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+--- Thread: Mag fed loads not grouping well (/showthread.php?tid=19303)
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Mag fed loads not grouping well - LoadHunter - 07-25-2019
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?
Mag fed loads not grouping well - lazyengineer - 07-25-2019
Thanks for the post, and this is very interesting. I'm curious to see what folk's replies are on this.
Mag fed loads not grouping well - VASCAR2 - 07-25-2019
Have you loaded the rifle using the charging handle and cycling the round into the chamber. Fire one round where a round is fed into the chamber. Extract that round to inspect for a change in cartridge overall length or damage to the bullets.
If the cartridge overall length is changing this could affect each rounds pressure, different pressure equals different velocity. At distances of 600 yards this could contribute to stringing. Are the bullets contacting the front top edge of the magazine during the feeding cycle? What barrel length, gas system, gas block (fixed vs adjustable) buffer weight and spring are you using? Are the bullets being marred by contacting the barrel extension?
Have you measured the first round from the mag for overall length, loaded the round then extract the round to check for change in cartridge overall length? The bullets/cartridge overall length in the magazine could be moving during recoil and through the feeding process changing the cartridge overall length. Out of curiosity what magazines are you using? Have you noticed this same stringing with different type of bullet or different reload or factory ammo.
Have you verified there are no clearance issues between the gas block and hand guard. Is there a muzzle device on the barrel?
I’d also want to make sure the barrel nut is tight and the front of the upper receiver is lapped. Have you verified the scope is torqued to correct specs and the mount is secure.
This is where I would start. If I assembled the upper I’d know whether the barrel nut was tight and the receiver lapped. I know Alexander Arms used to use Loctite on barrel nuts in the past but if someone else assembled the upper it is worth checking. It appears something is changing during recoil, you just have to isolate what is changing.
Mag fed loads not grouping well - LoadHunter - 07-25-2019
Thanks Vascar! Lots to chew on in your response, but that's what I'm looking for. I'll start the process of elimination now. I did assemble the upper. I trued the receiver and bedded the barrel with sleeve retaining compound. It was properly torqued when assembled, but I'll double check that. I'm running a standard flash hider and ASC 10 round mags.
Edited for brain fart...
Mag fed loads not grouping well - Gusmeister - 07-25-2019
Loadhunter,
That is an interesting problem. Thanks for sharing.
Personally I would try 2 things beyond what has been suggested (good stuff btw). You may what to chrono the load both single fed and magazine fed. If they are different you can be pretty sure the bullet is moving a lot. I learned on 300 BLK that crimp is not a decent substitute for neck tension. I never had that issue with the Grendel but you never know. The other thing I'd do is buy a box of Hornady 123gr ELD Black and go thru the comparisons again.
My guess is your bullet is not staying put after being chambered from the mag even though my Grendel brass has always had excellent neck tension. Good luck and please continue to share as you go.
Mag fed loads not grouping well - A5BLASTER - 07-25-2019
LoadHunter Wrote:Thanks Vascar! Lots to chew on in your response, but that's what I'm looking for. I'll start the process of elimination now. I did assemble the upper. I trued the receiver and bedded the barrel with sleeve retaining compound. It was properly torqued when assembled, but I'll double check that. I'm running a standard flash hider and ASC 10 round mags.
Edited for brain fart...
Check the front edge of your mag to see if the case mouth is making contact when loading.
Awhile back this was a problem with some asc mags. If it's contacting their it could be jacking up your neck tension and allowing the bullets to change seating depth.
Mag fed loads not grouping well - LoadHunter - 07-25-2019
Gus, single feed sd is in the single digits at 2449fps average. Ill see if that changes during magazine feeding.
I suspect neck tension as well. I have .003-.0035 of tension now with the hornady brass and .0035-.004 with Lapua. I may try to increase that somewhat. Im using hornady dies if that matters and I have polished expander balls in other dies in order to get appropriate neck tension but those numbers looked ok.
A5, Im seeing no damage to the cartridge hand cycling, but Ill take vascars advice and check a naturally cycled cartridge after firing a round.
Gus, I have some hornady black. Hopefully this evening I can try it.
If not ammo or feed related, then I fear Im looking at an assembly problem. I dread the thought of getting the barrel out with the sleeve retaining compound in place.
Im fortunate that my shooting range is just out back of my house but work sometimes gets in the way of play!
Mag fed loads not grouping well - Gusmeister - 07-25-2019
Hey Loadhunter,
Sounds good. Just trying to either eliminate or demonstrate possibilities.
Mag fed loads not grouping well - VASCAR2 - 07-25-2019
A5BLASTER Wrote:Check the front edge of your mag to see if the case mouth is making contact when loading.
Awhile back this was a problem with some asc mags. If it's contacting their it could be jacking up your neck tension and allowing the bullets to change seating depth.
That was a concern and why I asked which magazines were used. Good to know the receiver was lapped and you bedded the barrel. There have been reports of muzzle devices affecting accuracy especially if a lot of torque was used to time. Many forum members prefer to use a lock nut or shims instead of crush washer.
I think if it was a muzzle device causing accuracy it would be apparent at shorter distances.
If the Standard Deviation goes up with magazine fed ammo that makes me believe the bullets are moving in the mag or during feeding.
Another possible culprit could be the sizing die if neck tension is erratic or is over working the case mouth and neck. What sizing die are you using?
Mag fed loads not grouping well - VASCAR2 - 07-25-2019
A5BLASTER Wrote:Check the front edge of your mag to see if the case mouth is making contact when loading.
Awhile back this was a problem with some asc mags. If it's contacting their it could be jacking up your neck tension and allowing the bullets to change seating depth.
That was a concern and why I asked which magazines were used. Good to know the receiver was lapped and you bedded the barrel. There have been reports of muzzle devices affecting accuracy especially if a lot of torque was used to time. Many forum members prefer to use a lock nut or shims instead of crush washer.
I think if it was a muzzle device causing accuracy it would be apparent at shorter distances.
If the Standard Deviation goes up with magazine fed ammo that makes me believe the bullets are moving in the mag or during feeding.
Another possible culprit could be the sizing die if neck tension is erratic or is over working the case mouth and neck. What sizing die are you using? If your using bushing dies like Redding which bushing are using.
Mag fed loads not grouping well - lazyengineer - 07-25-2019
I will say, I thought I experienced this sometimes in my 6.5 Creedmoor AR10. I recall having some issues with some of my extractor tension for a while, and had to single load each shot for a few strings. There was vertical stringing with that particular ammo, and it went away went I switched to manually loading every shot; and I didn't know what to think of it. Most explanations on the topic, you'd think would show up as overall general group dispersion, not tight windage but large verticle dispersion. I figured it had to do with recoil moving the gun back shot to shot, which cased different recoil behavior the next shot, which affected it's elevation of impact. But who knows, I'm curious what others say.
Mag fed loads not grouping well - Klem - 07-25-2019
Loadhunter,
At this stage of the thread if you want to discount the bullet shifting prior to firing then it does not sound mechanical.
How many rounds are we talking about? Was this all on the same day?
Mag fed loads not grouping well - FLshooter - 07-25-2019
I agree with the advise given.Sounds like the bullet is being pushed in.First I’d do is determine if it does it with all mags or just that particular one.If it does it with all mags,I’d mic bullet length,mag feed,eject,then check length again.
I’d also try the same mag and ammo in another Grendel,if possible.
You need to use the process of elimination.
Mag fed loads not grouping well - LoadHunter - 07-26-2019
Today I tested Hornady Black eld against my AMax reloads at 500 yards (5 round groups). Single loading the black resulted in a group measuring approximately 7". Velocities were 2453, 2434, 2447, 2454, 2452. The mag fed group measured approximately 11" and the velocities were 2478, 2458, 2456, 2450, 2438 in that order. The 2478 hit considerably high. Without that shot, the groups were very similar. I checked each round after they chambered and noticed nothing unusual. Oal didn't change and there were no marks on the bullets or brass.
Reloaded single fed group measured approximately 5.5". Velocities were 2485, 2482, 2470, 2468, 2487. The mag fed group measured approximately 8" and velocities were 2469, 2479, 2478, 2485, 2482. I checked each round after they chambered. Oal increased .003-.006". There were no marks on the bullets or brass.
The mag fed groups were larger but not exceptionally so. The oal did increase but velocities remained in line with the single fed rounds. The groups I consistently got Tuesday were much more exaggerated than these today. I'll be firing many more rounds in the next few days trying to figure this out. I guess the 10 reloaded rounds today were just not enough.
I really appreciate the feedback I've received and will be checking all the variables that have been suggested. If anyone has an epiphany please don't hesitate to let me know.
Mag fed loads not grouping well - Klem - 07-26-2019
Loadhunter,
The reason I asked how many rounds you have fired is that when generalizing we're dealing with spreads across a Normal curve. If you fire one group of five rounds then it is only indicative of that group. It is easily argued that it is not enough rounds to be generalizable to all future groups. In this case it could just be how the wind was blowing at the time, or how the gun jumped during that string.
In your last post you acknowledge that '10 reloaded rounds were just not enough'. For some shooters that's a check-zero and fouling the barrel. I would fire a lot more than that before forming a vertical group opinion about the gun.
Mag fed loads not grouping well - Lemonaid - 07-26-2019
A few things to consider.
1. Test without the flash hider vs. with flash hider.
2. If using a bi-pod, shoot off sandbags. If shooting off sandbags try a lead sled. Evaluate how you are shooting, relaxed hold or holding with a fair amount of tension. Downward pressure from the pistol grip? excessive cheek pressure on the stock? etc.
3. Dry fire a number of times between groups and check for reticle bounce
4. Recheck all scope mount screw, scope ring screws and if using a quick detach setup, the tension of the locking levers.
5. Have someone you know who is good shooter with the Ar platform do the same test with your rig to confirm or invalidate the results.
You mentioned using crimp. No difference with vs. without? Was that with the Lee factory die or something else?
Best of luck!
Mag fed loads not grouping well - LoadHunter - 07-26-2019
During the first session I fired 5 or 6 groups of 5 rounds each and they all exhibited the extreme vertical spread when mag fed. I tried single loading during the same session and the vertical went away.
Yesterday I had a limited amount of time and noticed minimal vertical. You are correct that I need many more sessions to draw a definitive conclusion, but it was so obvious, repeatable and extreme Tuesday that it caused concern.
I have not yet conceded. I have brass cleaning as I write this, so testing will continue.
Mag fed loads not grouping well - LoadHunter - 07-26-2019
All good advice Lemonaid. Thanks.
Yes, I am using the Lee Factory Crimp die.
Mag fed loads not grouping well - LoadHunter - 07-26-2019
Oh, and I am shooting from sand bags. I am most comfortable and accurate doing so. Bipod prone will be attempted soon.
Mag fed loads not grouping well - Sinclair - 07-26-2019
LoadHunter, All of the advice to date has been around the weapon/load, there is the psychological aspect of shooting. I have seen this many times, mostly with pistol shooters. One man claimed that his .357 loads would hit the same POA at 25 and at 100 yds. He too had a range out his back door and shot there regularly. If you got off to the side and watched the recoil, it was much higher when he was shooting at 100 yds then at 25 yds with the very same ammo! He was anticipating the shot and adding the recoil. Are you expecting the vertical stringing because it happened once? Force yourself to relax, concentrate on how you are holding the weapon, sight picture, trigger squeeze, breathing. Shoot a string slowly as if each round is the first and only shot you are going to take. Just something to consider. The mechanical advice should be ruled out first, but our subconscious mind can do some amazing things.
As a disclaimer, I have never fired an AR, but if I understand correctly, they can be a little finicky about how you hold them.