Mag fed loads not grouping well

[Load Hunter]

I'm shooting a 16" AA fluted barrel with 6.5 Grendel bolt purchase at the same time from AA a few years back. I'm just now working up loads and have found a load using 123gr hornady over 27.7gr of ARComp to group 5 round groups under 1" consistently at 100 yards single loading as I normally do during load development.

I moved to 500 yards and shoot those loads into 5" without difficulty, but when I mag feed, my group vertically strings 10-12". Horizontally the rounds fall within a 4-5" spread. Manually cycling loaded rounds I've seen no damage to bullet or brass.

I've tried no crimp, light crimp, heavy crimp and lapua and hornady brass but nothing has helped.

What do I try next?

 

[CNJarvis]

I would suspect that the bullet is picking up some damage from the barrel extension lugs as the bolt pushes it from the magazine into the chamber. You might lock the bolt carrier back, then release it and let it chamber a round from the magazine, then look to see if there's any damage. Since the magazine is a double stack, the cartridge will be chambering on either side of the bottom center lug, so be sure to let it feed from both sides and check each round.

 

[Load Hunter]

I've checked the rounds by manually cycling and also after being fed from the mag during live fire. There is no damage to the bullet nor the brass. There is some increased OAL. +.003-+.006 but velocities remained consistent.

 

[FUNCTIONAL]

so if you load from a magazine and eject the live round you are saying the round GREW .003-.006?

I agree with bullet damage as being a first guess but if you are saying its growing I'd say you are jamming your bullets into the lands. Possible the different momentum of the bolt on single vs mag fed is enough to push the bullet differently into the lands and cause some issues.

If that is all true then I'd check a magazine fed live ejected round by the ogive and see if you see any rifling engraved. Weather you do or dont I'd still stuff the bullet in the case another .010" and see work a seating depth with that being your MAX btog length.

 

[Load Hunter]

All good advice. Thanks.

Yes, the OAL grew, measuring to the ogive, when mag fed during live fire and also manually cycling the action. I am loading 10 - 15 thousandths off the lands. But...the velocities still remained consistent.

 

[lash]

It’s not uncommon for rounds to grow when mag loaded in a semi. It’s about recoil and the force of a round being slammed into battery by the bcg. That’s why many semi shooters use more neck tension in their AR rounds than they do in a bolt. Some even use a light crimp.

Now, based solely upon what I’m reading, I don’t know that the slight oal growth is your issue. Absent any other information, I would look to the fundamentals of shooting a semi. There is a difference in how the rifle and recoil pulse acts between single feeding and mag feeding. There are even people that load one extra round than they need for a stage. Just so that the different recoil pulse of the bolt locking back vs. loading another round in the chamber is not a factor.

 

[Load Hunter]

Lash, that is something I am not familiar with. It is possible I am the cause of the problem, but since the single fed groups were so good, I assumed my fundamentals were alright. I'll look more into that.

 

[FUNCTIONAL]

It’s not uncommon for rounds to grow when mag loaded in a semi. It’s about recoil and the force of a round being slammed into battery by the bcg. That’s why many semi shooters use more neck tension in their AR rounds than they do in a bolt. Some even use a light crimp.

Now, based solely upon what I’m reading, I don’t know that the slight oal growth is your issue. Absent any other information, I would look to the fundamentals of shooting a semi. There is a difference in how the rifle and recoil pulse acts between single feeding and mag feeding. There are even people that load one extra round than they need for a stage. Just so that the different recoil pulse of the bolt locking back vs. loading another round in the chamber is not a factor.

If he loaded one in the mag and then dropped the bolt release and shot it loaded one at a time like that it should rule out that the magazine/bolt were causing any problems. If it starts opening up once he loads multiple in a mag it should be his shooting. Good point with the diffrent recoil impulse...will be interesting to see.

I had (maybe wrongly) assumed he was droping a round ontop of the mag or pushing it into the chamber and closing the bolt. Not having it strip a single round from the mag.

 

[RTH1800]

Get a Redding taper crimp die.

 

[FCS]

First and last round out of a magazine not shooting to same point as other rounds from a magazine is a well documented, phenomenon in gas guns. Well documented doesn’t mean well understood but it is a pretty well known / documented occurrence.

First round is being chambered at a different “pace” / impact velocIty of bolt contacting the base off cartridge in magazine, pushing up feed ramp and extractor sliding over the rim when bolt closes vs released by bolt stop / charging handle pulled to rear / released.
Last round you have no pressure on bottom of the BC that “lifts” whole assembly which changes how bolt / BCG interacts / interfaces with cartridge.
How over gassed system is also impacts BCG velocity and influences the first / last round out of magazine effect.

123 at 0.010-0.015” off lands will not fit in any magazine I have. Possible you have a shorter freebore as AA was getting barrels from Saturn that had a chamber geared more for 107 SMK @ mag length . What is OAL to touch lands w/123 in your chamber?

I’ve gone (about 2 decades back) to developing gas gun loads when loaded at OTM mandated OAL and fed from the magazine. Develop load in way platform will be used, otherwise you end up doing it twice.
Nada wrong with that, you have a load for single load and OTM use. I got lazy, do OTM loads for the gas guns and call it good. I have bolt guns if gas gun isn’t accurate enough.

 

[Ledzep]

Load a few further off the lands. I documented OAL growth of up to .015" in my Grendel. Tried neck sizing FL die with bushings .001,.002,.003, .004, and .006" undersized, no correlation. Growth averaged in my case about .008" IIRC. A crimp was required to eliminate growth, but it can be a very narrow window IME between enough crimp to hold, and so much crimp that it deforms the bullet and hurts accuracy.

Trying to seat .010-.015" off the rifling in a semi auto is on the close side. Between fouling and OAL growth you may end up on or very near the lands which causes drastic accuracy difference, even if pressure/velocity appears normal. I'd aim more like .025-.035" off the lands and see how that runs.

 

[Load Hunter]

If he loaded one in the mag and then dropped the bolt release and shot it loaded one at a time like that it should rule out that the magazine/bolt were causing any problems. If it starts opening up once he loads multiple in a mag it should be his shooting. Good point with the diffrent recoil impulse...will be interesting to see.

I had (maybe wrongly) assumed he was droping a round ontop of the mag or pushing it into the chamber and closing the bolt. Not having it strip a single round from the mag.
[/QUOTE

Yes...single loading directly into the chamber then dropping the bolt

 

[Load Hunter]

S

Load a few further off the lands. I documented OAL growth of up to .015" in my Grendel. Tried neck sizing FL die with bushings .001,.002,.003, .004, and .006" undersized, no correlation. Growth averaged in my case about .008" IIRC. A crimp was required to eliminate growth, but it can be a very narrow window IME between enough crimp to hold, and so much crimp that it deforms the bullet and hurts accuracy.

Trying to seat .010-.015" off the rifling in a semi auto is on the close side. Between fouling and OAL growth you may end up on or very near the lands which causes drastic accuracy difference, even if pressure/velocity appears normal. I'd aim more like .025-.035" off the lands and see how that runs.

Sounds like a plan.

 

[Load Hunter]

FCS, my chamber is shorter than many. OAL for the AMax/ELD is 2.275. Berger 120''s touch the lands at 2.228 OAL. Sierra match kings reach the lands at exactly 2.30" OAL.

 

[FCS]

I’d try 2.240-2.250 for OAL, adjust powder charge accordingly.
AR Comp might be a touch on quick side.

What velocity with 27.7 of AR Comp & 123 Amax/ELD?
Mid or intermediate gas system?
Buffer weight?

 

[Load Hunter]

FCS, I'm running a carbine buffer, mid length gas and I'm averaging 2450 fps with the 123 AMax's.