03-11-2020, 01:35 AM (This post was last modified: 03-11-2020, 01:41 AM by Lawrence97.)
Greetings,
New to the forum and I just built a 6 Grendel for an AR15 platform. I am using a 22 TBox barrel w a 1-8 twist and adjustable gas block.
I am not able to find anything that will shoot good with a 107 SMK. I started with 26gns of the following powders to get a baseline along with CCI 450 primers and Hornady brass sized with a .270 bushing and all rounds seated to mag length of 2.260: Varget, RL15, H4895 and AR Comp. Here are my chrono results for 5 shot groups:
Varget: 26gns (AVG 2518 ES 69 SD 25.2)
RL15: 26gns (AVG 2416 ES 76 SD 34)
H4895: 26gns (AVG 2606 ES 36 SD 13)
AR Comp: 25gns (AVG 2551 ES 72 SD 34)
All charges were dropped with an AutoTrickler v3 and A&D fx120i scale. Not sure how to post pics, but the groups shot w the above charges were all over an inch and nothing to be proud of.
I went out a second time with H4895 and AR comp to run the powder charges up and find the limits.
H4895:
26(AVG 2621 ES 69 SD 26)
26.5(AVG 2650 ES 53 SD 21)
27(AVG 2700 ES 70 SD 26)
27.5(AVG 2749 ES 33 SD 13) and (4 sighters at 26.5 and +.010 AVG 2654 ES 107 SD 49)
AR Comp:
25(AVG 2559 ES 63 SD 26.3)
25.5(AVG 2608 ES 45 SD 16.5)
26(AVG 2663 ES 50 SD 21)
26.5(AVG 2685 ES 36 SD 15) and (4 sighters at 25.5 and +.010 AVG 2590 ES 50 SD 21)
*26.5 of AR Comp showed pressure signs and is the upper limit.
I have an extensive background shooting fclass and PRS and I know that nothing is loose on my AR nor did I shoot in the worst of conditions.
Any ideas why this grendel is not performing? Or am I wrong in trying to make the 107s work? Open to any suggestions.
03-11-2020, 03:09 AM (This post was last modified: 03-11-2020, 03:13 AM by VASCAR2.)
It might be your barrel doesnt like that particular bullet or maybe the barrel is not actually 1:8 twist. Ive read that actual twist in production barrels might vary on their actual measured twist. Sierra states the 107 grain SMK needs at least a 1:8 twist. The 110 grain bullet needs at least 1:7 or faster twist. You might try the 100 grain tipped match king or the 100 grain SMK to see how they shoot before passing judgement on that barrel.
Another option might be the 105 grain Berger Target bullet but it does havev a little BC compared to the 107 grain SMK.
What range were you shooting those groups? If this is 100yds then it looks normal for an average barrel. TBox with bolt is at the top end of pricing however so I would expect it to perform as well as a Lilja, Bartlein or Kreiger. We don't hear much about TBox on the forum. Maybe the 107 doesn't like the fast 1:7.5 twist given the norm is more 1:8.
Your extreme velocity spreads will not be the fault of the bullet and may be something else like neck tension. The second group with two groups of two holes could be not enough neck tension in an auto loader. The cartridge slams into battery and the bullet continues on into the lands. This then causes a pressure spike and higher MPI. Do you know your neck tension (a common standard is .004" for auto loaders).
Regardless of the velocity data a couple of those groups look actually pretty good. Have you had much experience with an AR? It is certainly not an 8kg heavy barrel F-Class gun with 3" wide parallel tracking stock.
Klem,
The groups were shot at 100 yards. My barrel is a 1-8 twist. Neck tension used was .003. My experience with an AR is extensive with the platform as a .223.
I will look into adding neck tension and testing with a 105 VLD.
Not for the 6 Grrr but another 6mm (1:7.7), I found myself gravitating to the 105 HPBT hornady and 103 Eldx... even though a hunting bullet - not as twist sensitive. Sounds like you have a lot more experience than I but like Vascar my 2 cents' is maybe look at a couple other bullets.
"Down the floor, out the door, Go Brandon Go!!!!!"
Went out today and tested with more neck tension and the Berger hunting 105 vld. I used CCI 450s with H4895 and AR Comp. All bullets were seated to mag length (2.260). With the extra neck tension I had pressure signs with 26.5 of H4895 so that is the only charge I tested. All groups are 5 shots from 100 yards.
I do want to put in one other suggestion for you which you might already be aware of. In your ladder testing of this case, whose capacity is close to that of the normal Grendel, I would limit my gr-increases to 0.3 max, mostly I use 0.2, ie, slightly less than 1% of case volume. A 0.5 delta lets you cover a powder range, true, but you could very well miss any nodes by passing right over them, grain-wise.
"Down the floor, out the door, Go Brandon Go!!!!!"
03-11-2020, 10:34 PM (This post was last modified: 03-11-2020, 11:35 PM by Klem.)
Lawrence97 Wrote:Went out today and tested with more neck tension and the Berger hunting 105 vld. I used CCI 450s with H4895 and AR Comp. All bullets were seated to mag length (2.260). With the extra neck tension I had pressure signs with 26.5 of H4895 so that is the only charge I tested. All groups are 5 shots from 100 yards.
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Lawrence,
The neck tension theory still holds true given the information in your latest photo.
If you look at the vertical bias of the first two groups; the H4895 and the 25gns of Comp there's no reason for this as you are obviously a shooter. The next giveaway is the first round low of the second group. This will have been fed into the chamber under control and therefore less chance of the bullet moving in the neck. The rest of the group will have been auto chambered with more violence. Bullets shift forward to kiss the lands and the start pressure spikes. I have experienced this myself and am also an F-Class shooter.
The MPI's getting lower on the target as the loads increase defies physics (see attached MPI calcs). I have no clue why you are shooting progressively lower as you load more powder in, all other things being equal.
Your group sizes of around the 1MOA mark for 4 careful shots is average for an average barrel.
Choosing a different bullet would seem the path of least resistance but I doubt it is the fault of the bullet. Vertical bias will follow you to the next bullet and haunt you - the data is not pointing at the bullet.
The first group on the bottom row with the low shot was a round that I chambered by releasing a locked back bolt. I could not tell if the round had chambered so I ejected it and once again locked the bolt to the rear. I re-inserted the round into the magazine and let the bolt go forward. That round shot low and the rest shot higher. I will try this charge again and see if it repeats. From what I can tell, there might be something worth testing in the 25 - 25.5 gns of AR Comp.
I am using new brass. The new brass measures 1.190 w my headspace gage and after firing it is measuring 1.199~1.200. I dont think much will change for pressure tolerance with once fired brass. The biggest improvement was definitely adding more neck tension. I will run a quick test with this same neck tension and the Sierra 107 to confirm the poor accuracy was due to neck tension.
As for the point of impact (POI) going lower with increased charge weights is totally fine and this is what happens at short range. Slower loads will impact higher. This statement can be reversed if shooting at longer distances.
03-12-2020, 12:32 AM (This post was last modified: 03-12-2020, 01:08 AM by Klem.)
Lawrence,
The twin manual chambering does seem to coincide with a different MPI relative to all the other single manual chamberings or auto-loading. As you know, a bullet at the top of the stack sits above the forward wall of the mag so usual OAL limits do not apply. It could have moved forward twice and still able to be fed from the mag. Maybe a bullet sitting against your lands on start goes low, although the opposite was my experience.
If the bullet is moving on chambering the final OAL could also be shorter from being pushed back, twice as short for that particular round.
It's unsurprising that a new barrel displays higher pressures due to being a bit rough, or at least until a few firings have lapped it. The TBox may also be a tight barrel.
You list 30.5gns of water for the 6AR in your QL analysis. Is this correct for your cases? I don't shoot the 6AR but if you believe Wikepedia it's 36gns of case capacity, same as the Grendel 35-36gns. If it's a Grendel case necked down I wouldn't think there's a full 5gns difference. Certainly make a big difference in your pressure predictions.
Please share or provide a link to magazines that will allow seating longer in an AR15 platform. If there are some out there Id gladly purchase. I am currently using an E-lander magazine and it only allows 2.260 COAL.
