AR15 6.5 Grendel Proof Barrel Upgrade

[AZgeek]

Been struggling with this upgrade in the area of accuracy. I've successfully met my goals of lightening up the platform my virtue of the barrel and an ultra light Odin Works rail. I suspect this may have produced some untended consequences. I have added an SA adjustable gas block as part of this build but even with that, the recoil impulse seems harsh and the gun moves around to the point of staying on target is almost impossible without the weight of my left hand on the rail for stabilization. Seems to only like the heavier 123 to 129 gr bullets. Perhaps a better match for the 1-8 wist?

I've got a 7.62 Surefire muzzle brake I'll be installing that I found laying around. Certainly not the best but it will help. Just on old 308 bird cage on it now. I'm also wondering if a new buffer & buffer spring setup is in order?

Thoughts?

 

[AZgeek]

Picture of rifle:

 

[flyer]

Lighter guns recoil more.

 

[AZgeek]

In addition to the Surefire break, I'll be installing a CTS AKTIVE Recoil Buffer with anti-tilt and new spring. Got to wait at least three weeks for the buffer. Guess they are in demand. Decided not to go with the Armspec or JP SCS noise reduction is not my objective but rather reducing muzzle climb and creating a smoother recoil impulse. Hopefully this comb will work. Worse case, I'm looking at my first suppressor.

 

[Yondering]

I have added an SA adjustable gas block as part of this build but even with that, the recoil impulse seems harsh and the gun moves around to the point of staying on target is almost impossible without the weight of my left hand on the rail for stabilization.

That is a bummer. Try the brake, that should help some but might not be the whole formula needed. Are you sure the gas is adjusted right, i.e. tuned to just lock back on empty 5-10 times in a row and maybe a hair more gas for adverse conditions? What buffer weight and spring?

I do suggest trying one thing at a time, so you know what helps and what doesn't. Just shotgunning parts leads to a lot of confusion about what really works.

How much does your rifle weigh? It doesn't look like a super light rifle with that PRS stock; I guarantee it's heavier than my SBR 6.5 Grendel but that one is the flattest recoiling rifle I own. Balance might be part of the problem, IMO heavier in the front recoils better, yours looks like the weight is more to the rear.

Just for comparison, this little gun is also 6.5 Grendel, and is fairly easy to double tap 8" steel with at 100 yards, and easy to spot hits at the same or farther, just because it tracks flat. It uses a standard carbine spring and buffer, adjustable gas, and a medium-light 12.5" barrel with a lightweight Rogers SuperStoc at the other end, and a single port brake that I made. Forgive the nasty camo job please, I need to redo it.

 

[Yondering]

Also, can you tell where in the recoil cycle you're going off target? Is it the firing impulse, or the carrier returning forward?

Technique wasn't mentioned either and I don't know your abilities, but will assume you're straight and flat behind the rifle shooting prone, not off to one side or with your shoulder too high. Here's a quick pic I had handy that shows what I'm talking about for the most part (it's my buddy shooting the SBR pictured above). He's got the line of the rifle inline to his torso, feet spread and relaxed, and shoulders square to the target. His left shoulder is up too high, don't copy that part, but right after the pic he put his first long range shot ever (580 yards) directly on target and was able to follow up pretty quickly with a second hit.

Deviation from having the rifle in-line with your body will result in more muzzle jump and just generally wandering off target, either vertical or to one side, especially if you're slightly angled to the target.

Hope that stuff helps.

 

[AZgeek]

Also, can you tell where in the recoil cycle you're going off target? Is it the firing impulse, or the carrier returning forward?

Technique wasn't mentioned either and I don't know your abilities, but will assume you're straight and flat behind the rifle shooting prone, not off to one side or with your shoulder too high. Here's a quick pic I had handy that shows what I'm talking about for the most part (it's my buddy shooting the SBR pictured above). He's got the line of the rifle inline to his torso, feet spread and relaxed, and shoulders square to the target. His left shoulder is up too high, don't copy that part, but right after the pic he put his first long range shot ever (580 yards) directly on target and was able to follow up pretty quickly with a second hit.

Deviation from having the rifle in-line with your body will result in more muzzle jump and just generally wandering off target, either vertical or to one side, especially if you're slightly angled to the target.

Hope that stuff helps.

Have not spent anywhere near enough time trying to figure this all out but I'll try and answer some for your question. Been out 4 times now with the new build. First time out was really just getting the adjustments with the gas block set and making sure there were no functional issues. All went very well.

Second time out I was shooting my tried and true loads developed on my old Grendel upper. FYI my old upper is much heavier and has a very effective break on it. It also has a slower twist rate of 1-9 vs 1-8 on the new Proof. Chalked the shitty groups up to just barrel break-in, getting use to a new trigger and bi-pod. I've only been bench shooting thus far - no prone.

Third time out I was scratching my head. Could not make heads or tails of why I was not achieving the accuracy I got out of the old upper. I was also trying an array of different bullets and weights with nothing standing out. Typical group sizes where about 1.75". I've been shooting Grendel since 2006 and know the round well. My old upper shot between .75 and 1" consistency with very little effort.

I was this last time out was when I noticed the more violent recoil impulse, muzzel jump and the drifting of the gun to the right on the bench. To your point, I have to be very careful with repeatable alignment and technique now. Its just harder to shoot this new build. Since my son won't trade for my old upper back, got no choice but to figure this out. I don't know the weight off the buffer or type of spring. Never had a reason to care until now. I'll need to pay more attention to determine if movement is associated with the firing impulse or carrier forward.

 

[Yondering]

Interesting. Some rifles are definitely more sensitive to technique, especially when the recoil is harsh and sharp. Good to know you've been shooting the Grendel for a long time and had a good load on your previous upper, so that takes some of the variables out of the question.

I'm not sure I have any more suggestions at the moment, but will stay tuned, I'm interested to hear if you get it worked out and what the solution is.

 

[AZgeek]

Had a chance to get to the range today with the new Surefire break installed. Shot 7 different bullets. The results of six shown here:

Five shot groups at 100 yards. The Nosler NBT is a 10-shot group. The Nosler ABLR clearly shot best both times I've tried them. I think the NBT 100 has potential. I'm sticking with hunting bullets so my testing going forward will include the 129 ABLR, 123 SST & the 100 NBT.

The break helped but I think I still have to find the right harmonic load for this barrel. Thought these Proof CF barrels are supposed to be super stiff but maybe not. I'm hoping to have a CTS Active Buffer and new recoil spring installed for the next outing but either way I'll be doing reduced load ladder testing.

 

[AZgeek]

Well going from bad to worse. I'm thinking about using "down grade" instead of "upgrade"in the thread title. I reduced loads for the 129 ABLR, 123 SST and 100 NBT. Everything got worse except for the 100 NBT. A longer seating depth seemed to help with the 100 NBT in addition to a .5 gr drop in powder charge.

The only bullets that seem to hold promise are the 129 ABLR and the 100 NBT. This makes no sense as the sweat spot for the Grendel should be 120-123 grain bullets. This barrel just hates them! I went to the range yet again this morning but too crowded with yahoo hunters getting ready for dear & elk season.

My last testing effort includes only two load: 129 ABLR at a max load of 31.0 grains and a re-test of the 100 NBT with 29.0 & 28.5 grains of IMR 8208. I did load up 10 rounds each of the Hornady 123 SST and ELDM with 28.2 grains of 8208 just to rule out CFE 223 as a possible culprit. No Grendel IMHO should be this picky. If this hold true and I find no other technical reasons for the tuning problems, certainly would NOT recommend this CF barrel.

FYI. Still don't have the CTS Active buffer yet but probably best to finish load testing before the install.

 

[Pusher591]

Just curious have you tried bedding the upper and barrel? May be somehting to look into

 

[AZgeek]

Pusher - No I have not. I'll do some searches here and find out what's involved. Maybe just hand this off to a 3rd party to check my build work?

Strange as this build has functioned flawlessly. I had another guy I know at my local range shoot 10 round of the 100 NBT (with 29.5 gr of 8208 XBR) and his two 5-shot groups measured exactly the same as mine.

 

[Pusher591]

Pusher - No I have not. I'll do some searches here and find out what's involved. Maybe just hand this off to a 3rd party to check my build work?

Strange as this build has functioned flawlessly. I had another guy I know at my local range shoot 10 round of the 100 NBT (with 29.5 gr of 8208 XBR) and his two 5-shot groups measured exactly the same as mine.

Yeah I’m building a budget style precision ARamd I will bed mine. From what I’ve gathered I haven’t seen it make anything more accurate but more consistent which can in turn produce better accuracy. My Smith has had some “trouble” AR’s come in and after checking over the work and finding nothing he beds the extension and upper. It consistently helped with fliers which seems to improve accuracy consistentcy

 

[Mike_Honcho]

Bedding = square faced upper + locktite - correct?

AZ - you're harshing my fantasy groove wherein I build a super accurate lightweight 6.5G with a Proof that shoots the lights out.....

 

[Pusher591]

Bedding = square faced upper + locktite - correct?

AZ - you're harshing my fantasy groove wherein I build a super accurate lightweight 6.5G with a Proof that shoots the lights out.....

Yes from what I’ve seen, squaring the face and using red high temp loctite 272. Even seen some use rocksett but not sure you’d ever be able to get that off.

 

[AZgeek]

LOL. We share the same fantasy! I'm very happy with the weight reduction part. Cautiously optimistic about this next round of load testing and trying to keep the faith. But as my shooting bud always says, remember the "Riddle of Steel " from Conan The Barbarian!