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 Post subject: 556 DMR Range Report
PostPosted: Wed Oct 13, 2010 1:26 pm 
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Joined: Thu May 27, 2010 1:10 pm
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Location: Omaha, NE
I posted this at another forum a little while back, so I apologize to those who may have already seen it. Just thought it may be of interest here as well (really, I'm just trying to keep up with Skypup-LOL). The purpose of this test was to determine the accuracy of the Sig Sauer 556 DMR using increasing bullet lengths. A range of information can be found regarding the accuracy and ammunition (bullet weight) preference of this weapon system. This question arises due to the 1 in 10" twist rate of the 21" barrel, which may not be capable of handling longer (heavier) bullets. I therefore decided to determine for myself how my DMR performed.

Equipment

Weapon: Sig 556 DMR, 21" heavy barrel with a 1 in 10" twist rate, aluminum Quadrail foregrip, Magpul PMAG 30 MAGLEVEL magazines (Figure 1)
Optic: Nightforce NXS 3.5-15 x 50 (F2), Zero Stop, Mil-Radian turrets, MLR reticle
(setting = 15X)
Optic Mount: LaRue Tactical SPR/M4 1.93" QD (LT-135)
Benchrest: Caldwell Lead Sled Solo with 25 lb. weight bag


Image
Figure 1. Sig 556 DMR weapon system.



Barrel Break-in

Prior to range testing, the DMR was put through a barrel break-in procedure using 200 rounds (total) Black Hills 52 gr BTHP Match ammunition as follows:
1) 5 x 1 rounds; after each round, the barrel was cleaned with 10 patches soaked in Bore Tech Cu+2 Copper Remover, followed by 3 dry patches. The barrel was then treated with 3 patches soaked in Kano Kroil Penetrating Solvent, followed by 3 dry patches.
2) 5 x 3 rounds; each group of 3 rounds followed by the above cleaning procedure.
3) 5 x 10 rounds; each group of 10 rounds followed by the above cleaning procedure.
4) 2 x 25 rounds; each group of 25 rounds followed by the above cleaning procedure.
5) 1 x 30 rounds; followed by the above cleaning procedure.
6) 1 x 50 rounds; followed by the above cleaning procedure.

After the break-in procedure, the weapon was cleaned using a nylon bore brush and patches with Hoppe's 9 solvent, and lubricated with Hoppe's Elite gun oil prior to storage.

Ammunition Test

To carry out this test, I fired 20 rounds of each type of ammunition listed in the table below on semi-automatic setting (the DMR is also capable of non-cycling single shot fire). For each type of ammunition, I fired 5 shots at a range of 100 yds to sight in the weapon. I then fired 3 groups of 5 shots, allowing the barrel to cool several minutes in between groups. After each type of ammunition, the barrel was cleaned and lubricated with Hoppe's 9 solvent and Elite gun oil prior to testing the next ammunition. A benchrest was employed to minimize the effect of operator skill (or lack thereof) during the test.

For each ammunition, I determined the maximum hole spacing for each 5 shot group (measured center to center). The three group spacings were then used to determine the mean ± standard deviation (SD, n = 3) for each type of ammunition. "Flyers" (and there were a few) were not eliminated from any group measurement. I also shot the Hornady 55 gr SP each of the three days as a "control" to assess variation between days.

Weather Conditions

Day 1 Day 2 Day 3
Temperature: ~87 °F ~ 90 ° ~ 90 °
Wind: N, 3-5 mph N, 5-8 mph N, 3-5 mph
Altitude: 1095 ft 1095 ft 1095 ft
Barometric pressure: 30.12 R 29.92 R 28.84 R


Results

The individual 5 shot group data as measured in inches and mean for 3 groups ± SD are shown Table I. The velocity data shown are from the manufacturer, not chronographed from the DMR (I didn't have a Chrony at the time). Also shown are the mean values ± SD following conversion to MOA (÷ by 1.0472). Based on these data, there was not a dramatic trend with regard to bullet length and accuracy in the Sig 556 DMR. In the most liberal (I apologize for using the "L" word when talkin' rifles) estimation, the DMR accuracy generally improved as bullet length increased. However, the results obtained with Hornady 55 gr SP ran counter to that notion. What can be stated with reasonable certainty is that this weapon is capable of ~1.0 MOA accuracy at 100 yd. It should be noted that there were frequently "flyer" rounds in some of the groups. Whether this was due to the load or the operator is unclear. However, if these "flyers" were not counted, the accuracy of this weapon system would be solidly in the sub-MOA range.

Because the 21" barrel 1 in 10" twist rate of the Sig 556 DMR might be expected to handle longer (heavier) bullets poorly, it was surprising to find that the two of the better groupings were achieved using 77 gr loads. These results are unlikely to be an artifact as 4 different 77gr loads (Black Hills, Federal, Remington, Silver State Armory) fell in the 1.0 to 1.2 MOA range. The next closest loads in accuracy were the Hornady 55 gr SP and Black Hills 68 gr. This was also somewhat surprising in that I fully expected fate to dictate that the most accurate load(s) in my hands would also be the most expensive. Costing under $30.00 per box of 50 rounds, the Hornady 55 gr SP was the least expensive of any of the SP or HP ammunition used in this test (ie. good plinkin' ammo).
The least accurate groups were obtained using Federal XM193 FMJ, Fiochi 62 gr FMJ, and Black Hills 75 gr. None of these loads were able to produce an average grouping of less than 2.2 MOA.


Table I
Image




As an example of some of the groupings listed numerically in Table I, selected target images from the 100 yd test including the best single group, the worst single group, and the best 77 gr group are shown below in Figure 2.


Image
Figure 2. Selected Single Group Images.



In addition, for those (such as myself) more inclined toward pictures than text, I have also included the MOA data from Table I in a graphical format (Mean MOA +/- Std. Dev.) as shown in Figure 3. Most notable from this graph is the general indication that accuracy with the DMR is improved using bullet weights (lengths) of less than ~60 gr or greater than 64 gr. Based on the 1 in 10" twist rate, I had guessed prior to testing that optimal accuracy would be obtained from loads in the mid 60 gr range. Surprisingly, the 60 to 64 gr loads were among the least accurate of all the ammunition tested, validating the notion that an actual range test can be significantly more valuable than a prediction based on a specific twist rate. Had I not tested the various ammunition, I might have only tried the 60 to 64 gr loads and concluded that the DMR was inherently inaccurate.


Image
Figure 3. Graphical representation of Sig 556 DMR accuracy with various ammunition.



In light of the somewhat unexpected results with regard to bullet length and accuracy, the next phase of this experiment was to conduct the same test at 200 yd and 300 yd distance using the 2 most accurate heavy loads as determined above (Black Hills 77 gr, Federal 77 gr ). The rationale for this test was that 100 yd may not have been sufficient distance for the low spin rate imparted by the 1 in 10" twist barrel to adversely affect stability of longer bullets. Therefore, accuracy was determined at longer ranges to assess whether the Sig 556 DMR was still capable of stabilizing these longer bullets.

As shown in Table II, the 556 DMR appeared fully capable of stabilizing these rounds effectively out to 300 yd. In fact, 2 out of 3 groups using the Federal 77 gr at 300 yds had 3 of the 5 shots within a spacing of less than 1 inch. Therefore, if the flyers had not been included, this would place the DMR accuracy close to 0.3 MOA. The same was true for the 200 yd groups. How much of this was due to the shooter is unclear at this point. It will be interesting in the future to conduct a similar test while clocking the muzzle velocity of the rounds, to see whether any "flyer" correlate with abnormal muzzle velocity compared to other rounds. What is clear is from this test is that the DMR can effectively stabilize loads up to 77 gr at ranges out to 300 yd, while maintaining close to 1.0 MOA accuracy. Further testing with these loads at 400 and 600 yd will be carried out at a future date.


Table II. Groupings for Black Hills and Federal 77 gr Ammunition at 200 and 300 yds.
Image



Finally, what I predicted (but hoped wouldn't come true) at the outset of this test came to pass. With the Federal Gold Match 77 gr getting the nod as best overall ammo in the DMR, it was the most expensive (~$1.53 per round). Oh well, I had a lot of fun doing this test, so it was worth every penny. Further, if I decide to get into the reloading end of things, I now have a much better idea of where to start for creating custom loads optimized for the DMR.

As an update to this report, I have tested Black Hills 77 gr ammunition at 400 yd, and the groups started to open up slightly (average was 1.5 +/- 0.3 MOA). I am undecided as yet whether this means that as the range increases, accuracy will drop off as stability decreases due to sub-optimal twist rate. Unfortunately, while cleaning the DMR after the 400 yd session, I dropped and accidentally stepped on the gas tube. This caused a catastrophic failure in said part, on which I am currently awaiting to be repaired). Pretty bad mistake on my part. In any event, I will post more data from 400-600 yd after the repair is completed.


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 Post subject: Re: 556 DMR Range Report
PostPosted: Wed Oct 13, 2010 1:50 pm 

Joined: Fri Sep 24, 2010 6:21 am
Posts: 86
Outstanding field report!


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 Post subject: Re: 556 DMR Range Report
PostPosted: Wed Oct 13, 2010 1:55 pm 
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Wow, what a great write up and beautiful rifle! :D

Getting almost 2,800fps from the 75 grain heavy OTMs is nice.

I once had my sights set on a nice DMR but I did not think the 1:10" twist 21" barrel would stabilize the heavy OTMs, so I got a custom made 20" 1:7" twist barrel for my Thompson Center Contender rifle instead.

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 Post subject: Re: 556 DMR Range Report
PostPosted: Thu Nov 11, 2010 4:25 pm 
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Location: Northeast,Wisconsin
Outstanding writeup. Very professional. :D


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 Post subject: Re: 556 DMR Range Report
PostPosted: Fri Nov 12, 2010 11:47 am 
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Location: Omaha, NE
Thanks - I appreciate that! I'm going to prepare a dope chart for the DMR this weekend with FGMM 77 gr ammo at 100, 200, 300, 400, 500, and 600 yd to hopefully get everything "dialed in". I'll try to post the results when I have them.

Edit - I should also note that about a month ago, I replaced the LaRue LT-135 mount on the 3.5-15x50 shown in the picture to an LT-104, which puts the glass almost 1/2" closer to the receiver. I originally used the LT-135 because the DMR came with the upwardly-bent charging handle, and the shorter scope mount dramatically interfered with cycling the bolt. I replaced the original handle with the new flat handle and picked up the LT-104. The difference in cheek-weld with the shorter mount is like night and day compared to the LT-135. I also added an Accushot rear monopod to the PRS stock. I've been working with the modified setup for a few weeks now and I really think these few simple mods have made an enormous improvement to the system.


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 Post subject: Re: 556 DMR Range Report
PostPosted: Sun Nov 14, 2010 10:59 am 
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Excellent write up, thanks for sharing it with us. :)

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 Post subject: Re: 556 DMR Range Report
PostPosted: Tue Nov 16, 2010 11:53 pm 
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Joined: Wed Feb 24, 2010 2:28 am
Posts: 85
Location: Sunny Central Oregon
Outstanding field report! Thanks for sharing it with us, and the graphics are excellent too. It makes for a very good read with visual interest and all.

I have a few questions for you.

1) Is that a Harris bipod, and which model?

2) How do you like the Acu-Pod? Are they as great as they look/sound? Worth the coin?

3) Is there any chance you will do a report on the Sig 556? I'm really curious how well they would do if tuned for long range. Could they reach out to 800 yards? 1000 yards?

4) How did you determine the mv (velocity) of the ammo? Do you reload your own ammo?

Sorry, I don't mean to bombard you with questions, I'm just thinking of getting into reloading and I'm curious if you chrono-ed the ammo, or just looked up the ballistics of the mfgs. I'm guessing that you don't reload, or else you would have worked up your own loads for that rifle. But maybe you used to reload and don't have the time anymore, I'm just curious. In any case, very good work.

Thanks again!

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 Post subject: Re: 556 DMR Range Report
PostPosted: Wed Nov 17, 2010 3:52 pm 
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Joined: Thu May 27, 2010 1:10 pm
Posts: 196
Location: Omaha, NE
MeridianNW-I thought I had posted the 556 SWAT data here, but I guess I did not. I might as well put it in with the DMR range report (it's as good a place as any and it will all be in one thread in case anyone else is interested like yourself). I have addressed your other questions down below at the end, hope that helps


Two Sig 556 SWATs (16" barrel, 1 in 7" twist) were put through a barrel break in procedure as previously described for the Sig 556 DMR. Using a Nightforce 3.5-15x50 NXS (F2) scope with a LaRue QD mount and a Lead Sled Solo, 25 rounds of each ammunition were tested at 100 yd. The first 5 rounds were used to sight in, followed by 4 x 5 round groups. A Chrony Beta was used to measure muzzle velocity for 10 shot strings (placed 10 feet from muzzle per the directions). All ammunition tested except the Hornady 55 gr SP were BTHP Match grade. Weather conditions were excellent (sunny, 80 °F, very little wind).

The maximum spread for each 5 shot group was measured (center to center) and the mean ± standard deviation determined in inches and MOA as shown below:

556 SWAT #1
Image


556 SWAT #2
Image


As can be seen from the two tables, SWAT #2 seems to print slightly better groups with some of the ammunition tested. However, I would characterize the overall accuracy of the two 556s as about the same. I feel pretty comfortable concluding that these 556s are going to group in the 2.0-2.5 MOA range, possibly a little better, depending on the specific ammo used.

Other questions:

1) It is a Harris BRM on a LaRue QD mount

2) I LOVE the Accu-shot monopod, and it is absolutely worth the coin IMO.

3) Can a 5.56 reach out to 800-1000yd? Absolutely. I had a long conversation with a fella that is a long standing member at my range that is big into long distance AR shooting. He has an AR with a 28" barrel that he put together that apparently can routinely do 1/8 MOA groups in his hands @ 100yd (this was confirmed by several other members). He shoots this in distance competition from 600 to 1000yd.

But can a 556 SWAT reach out to 800yd? Not in my hands, that's for certain. At the 2.5 MOA accuracy shown above, you're talking (mathematically) about keeping 5 rounds inside a 20" circle at 800yd under absolutely perfect conditions. In reality, I doubt I could do better than 3 ft at that range. So maybe I would have a decent chance of hitting a vehicle-sized target at 800yd, but a man or animal-sized target? I wouldn't waste the ammo. Put another way, at 800yd with a muzzle velocity of 2518 fps, the Federal GMM with a 77gr SMK will have a drop of very close to 300 inches (that's 25 feet or 10.4 mil). I don't view that as a real high probability shot in my hands. Don't get me wrong, it can definitely be done with the right weapon and the right driver. But for me personally, I don't view the 556 (SWAT or DMR for that matter) as an 800 yd weapon. Part of that is due to the limitations of the weapons, part is the limitations of the cartridge. If I need to go out to that distance, I'll do it with a .308. In my hands, I would realistically say that the 556 SWAT is a 300yd weapon, and the DMR about a 500yd weapon.

4) I don't reload, and the MV data was recorded using a Shooting Chrony Beta chronograph.


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 Post subject: Re: 556 DMR Range Report
PostPosted: Fri Nov 19, 2010 1:15 am 
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Wow, another great write up, thank you gstaylorg!

I really appreciate that work you've done here. As much as I would like to get into reloading, I could also take the easy road out and just use say, the Federal 77-grain for precision shooting, and skip the reloading hobby all together.

This is an excellent reference for us all. Thanks again!

:P

And I think there is an Acu-Pod in my future as well.

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 Post subject: Re: 556 DMR Range Report
PostPosted: Tue Feb 08, 2011 11:32 am 
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Fantastic posting! Thank you for officially putting the myths that a 1-10 21" barrel is not sufficient for stabilizing the heavy rounds to bed. I can't wait to get mine out and break it in. Have you attempted single loading (bolt action style) 80-100 grain hand loads through your DMR?

Also, I thought I remembered reading in another thread that you use(d) a fish-gil handguard. Your photo, and part discription seems to suggest otherwise. Did you end up swapping it for the swat quad-rail for better stability? I was actually thinking of ditching the quad-rail on mine to save weight, and attempting to eliminate the play on the fish-gill. I'm curious to know though if you have already attempted this and just ended up defaulting to the quad-rail?

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 Post subject: Re: 556 DMR Range Report
PostPosted: Tue Feb 08, 2011 12:12 pm 
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LKY13,

Although I shot it a few times with the fishgill, I went to the Quadrail as soon as I could get one for the main reason that I wanted to attach a Harris bipod using a LaRue QD mount. This type of mount would have eventually (probably sooner than later) made a mess of the lower plastic rail, so I got the Quadrail.

As far as the twist rate goes, my feeling is that although the DMR seems to do reasonably well with the 77 gr loads in my hands, but I think that those pills are really pushing that barrel/twist to its limit. I've shot it a number of times out to 400 m (actual distance at my range is right at 427 yd) with BH and FGMM 77 gr loads. At the time I posted that report, I was not aware that that particular berm at my range was measured in m distance. Since finding that out, I have verified the distance at 427 yd with an LRF, so the accuracy turns out to actually be just a tick better than 1.5 MOA.

Having recently acquired a very accurate .308 with a great trigger and done a fair amount of shooting at the max distance of my range (600 yd), my take on my earlier shooting with the DMR is that it's probably a better shooter than what my numbers indicate. Part of that is certainly due to my mediocre trigger control, but part is also likely due to the fact that it has an 8 lb trigger pull, which is a little on the stiff side. However, I seriously doubt that the 21" 10-twist barrel is going to do very well stabilizing bullet lengths for projectiles in the 80 gr and above range. It's just really not optimal for those bullet lengths. It will do ok out to 400 m with the 77s, which was a pleasant surprise for me, but I suspect if you tried 80s or 100s, you'd either get keyholing, or at a bare minimum, the accuracy would be terrible. That barrel is just not intended for projectiles that are that long.

Then again, WTF do I know? I didn't expect the DMR would handle anything above about 69 gr very well, so there you have it (LOL). Obviously, the best way to find out would be to just shoot a few rounds and see. I don't reload and don't have access to handloads in the range you're talking about, so I've never done it myself. The other question would be whether you could even fit loads of that length in the chamber. As I said, I'm not a reloader, so I can't answer that question, but I'm sure there are others here that would know. I have fired the DMR with 77 gr loads with the gas valve in the non-cycling position (position 2, single shot mode) and it seemed to do just fine, but not any better than in semi-auto mode in my hands. That is part of the reason I think the 8 lb trigger pull is a limiting factor in the accuracy of the DMR, because in theory, it should shoot a little better in single shot mode. I suspect that someone with more skill and experience could drive the DMR much better than 1.0 MOA. But for 80-100 gr loads? I think that would be asking way too much of a 10-twist barrel. Had it been up to me, I would have gone with an 8-twist for the DMR and called it a day. But it is what it is and I've had a lot of fun with it, so I'm very happy that I got it.


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 Post subject: Re: 556 DMR Range Report
PostPosted: Thu Feb 10, 2011 1:12 pm 
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Once again, I appreciate your insight. It's interesting that the Swiss favor the 1 in 10 twist. I have access to reloading equipment, so perhaps I can give the heavier loads a go down the road a ways. I purchased the DMR to satisfy both the scientist and the sig lover in me, and intend to have some fun experimenting with different loads. At one point I was even considering a conversion to 6.5 Grendel. However I think I will be plenty happy with the 5.56.

I also have a .308 (also a 21" barrel 1 in 11 twist) and it will be interesting to compare the two. I have tested heaver loads that had to be single loaded in that caliber, but have only had mixed results so far. Eventually I *might* work up to purchasing the new 716 when they start selling the DMR version. I just wish that it was an evolution of the Swiss piston design(s) and not an adaptation of an AR-10. Not to get off topic, but what .308 did you add to your collection?

One last question on the fish-gill; did you have any accuracy issues with yours before the swap? I know some have mentioned a fair amount of play in that design. I didn't notice a problem while shooting a friends 556, but his was not the DMR, and I wasn't attempting to bench-fire for accuracy. While I want my rifle to be as light and short as possible I don't want to be switching back to the swat rail down the road after realizing the fish-gill is sub-par.

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Bundesgrenzschutz G1 FN FAL
1911 GSR Revolution
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P229R DAK
556 DMR
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 Post subject: Re: 556 DMR Range Report
PostPosted: Thu Feb 10, 2011 5:35 pm 
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Location: Omaha, NE
Here's my thoughts on the style of handguard and accuracy, opinions may vary. Are you familiar with the concept of free-floating a barrel? My thought is that "free-floating" is an oxymoron when you're talking about a Sig-flavored piston sytem. It CAN'T be fully free-floated. No matter what you do, the gas tube, piston, and gas valve assembly, are going to be locked into the gas block (along with whatever style of handguard you choose), which is in turn locked to the barrel. So technically, I don't see how a Sig can ever have a fully free-floated barrel. Having said that, the more strain and/or torque you apply to the gas block and/or barrel at some distance away from where the barrel attaches to the receiver, the more it may have an impact on accuracy.

The real question is, what is the largest possible source of strain/torque in the case of a Sig? I suspect it is the reciprocating piston action during the process of firing it, much more so than any effect that may be caused by a loose-fitting handguard under normal firing conditions. Remember, your glass is attached to the receiver rail, which is ultimately where the barrel is attached (the receiver, I mean). So even if the handguard moves a little, as long as you optic remains centered on the POA, it should go where the glass is aimed. My DMR's original fishgill handguard actually had noticeably less play in it than does the Quadrail, probably because of how the plastic tabs "snugged" it in at the gas block end. I did not notice any appreciable difference in the accuracy between the two handguards, although I did a much more thorough job of testing it with the Quadrail than the fishgill. My feeling is that the type of handguard is not likely to be the major source affecting accuracy in a piston-driven Sig 556. But again, YMMV.

Of course, there are lots of factors that could impact what I just said. For example, are you using a bipod attached to the handguard? Do you have the weapon "locked" down into some type of shooting platform on a bench, like a Lead Sled? How loose is the handguard in actuality? Finally, do any of these possibilities add up to more of an effect on accuracy than the driver's actual shooting skill? Maybe, maybe not...depends on the driver.

I think in the end, I'd say just get out and shoot it and see how it performs. If there is a big difference, you'll probably notice it pretty quickly and can take steps to correct the issue at that point, if necessary. Recall that the main reason I switched to the Quadrail was that I was concerned my QD bipod mount would chew the plastic on the fishgill, not because I was worried that the fishgill hindered the accuracy/performance of the DMR. Good luck with it!

BTW: my .308 is this:

viewtopic.php?f=17&t=10263


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