last round in AR mag ruins groups.

[twoatlow8]

Has anyone encountered this phenomenon? I am shooting handloads from my Barnes Precision AR that is wearing a 20" Krieger tube. I am trying to get ready to start shooting Designated Marksman Matches. My groups (10 rnd) are measuring in the high 0.4s to low 0.5s until the last shot. The last round gets thrown either low or low right. Anyone else encounter this?

Thanks, Joe

 

[fredhorace77]

if ur thinking it might be associated with the magazine and the last round try loading just 4 rounds and see if it still throws the last round out of the group. if it doesnt then it may be you pulling that last shot or the barrel maybe fouling alot by the 10th round. but could also be several other factors.

 

[twoatlow8]

It doesnt seem to matter how many rounds is in the mag. Just the last one flies. I have read that the benchrest guys go to great length to ensure that the recoil travel is the SAME for each shot. I always know when I am empty with an AR cause the recoil "feels" different. Wonder if this causes my prob?

 

[Beau]

Check the headspace on your loaded rounds. Shouldn't be more than about 3 thou less than fired cases.

Then take some fired cases and start sizing them down. Size them down until they will chamber & eject easily, using the slow, light pressure of the forward assist to chamber them. Measure the headspace on the cases. Then insert the cases into the chamber & slingshot the BCG onto them. Measure again. The headspace now shouldn't be more than 2 thou less than your first measurement. You'll want them lightly lubed when you are doing all this.

 

[jrm850]

I don't know the exact weight of a bolt carrier group so I can't do the math, but I suspect that the projectile will exit the muzzle long before the carrier starts to return to battery, so we can probably rule out the bolt impacting the bolt stop on the return as a cause. My first thought would be that the last round is getting dinged up a bit because it does not feed as well the others. However, always hitting low in the same spot supports the recoil theory more than the bent bullet theory. Maybe look at the difference in Carrier drag on the mag follower vs carrier drag on a brass case? Does it do this with different brands of magazines? Interesting problem.

 

[Dave in WI]

Maybe a stupid thought, but is loading an additional dummy round at bottom of mag an option?

 

[Hal]

Joe

A guy that post on here builds high end 45's. He said the point of impact of the group would moved depending on which clip he used.

Hal

 

[Warren Dean]

"Magazine"....not "clip"...clips go in Enfields and Garands.

 

[CatShooter]

"Magazine"....not "clip"...clips go in Enfields and Garands.

And Springfields and 40XB's Match, and... whoops, there are clips to load the magazines on M1 Carbines and M-16/M-4s and... Hummmmmmm?????

 

[M-61]

Just a thought.....let someone else load your magazine...their choice with how many.
You'll have no idea when the 'last' shot is coming. See if the same thing happens.

 

[Hopkins]

if you drop the magazine before you fire the last shot does it also shoot low or low right? a round or rounds in the magazine may alter the battery of the chambered round and bolt enough to alter the groups poi

 

[people]

Just a thought.....let someone else load your magazine...their choice with how many.
You'll have no idea when the 'last' shot is coming. See if the same thing happens.

It is amazing how this affects the shooter. Flinching just might be your problem. A snap cap in the mag can also tell you much.

 

[twoatlow8]

I try not to count as I shoot. That might creep into my head unknowingly at times though.

Hopkins, your comment will be a good diagnostic test. Thank you.

Beau, Will check this also. Thanks as well.

Joe

 

[brian427cobra]

The last round will always be a flier in AR platforms, the reason why is the bolt carrier doesn't have a round pushing up on it

 

[people]

The last round will always be a flier in AR platforms, the reason why is the bolt carrier doesn't have a round pushing up on it

Not in all AR's.

 

[MrMajestic]

https://www.youtube.com/watch?v=4NwP3wes4M8

;D

 

[mac86951]

I'd say try a different mag, or check for something dragging like the Bolt hold catch, causing a feed cycling issue.

shoot a 10-shot group with only 2 rounds in the mag each time.

-Mac

 

[fredhorace77]

the thought of the bolt not having to chamber a round with another one below it may cause the bolt to cycle harder/faster, could this be causing the bullet to slam harder into the feed ramp and causing ur bullet to seat further into the case, which would cause a change in pressure/point of impact?

 

[MrMajestic]

Try a Bob Sled.

 

[chuckbuster243]

I had the same thing happen. I can shoot them using a single shot follow and no change in impact. But with a mag last round was out of the group. So I just made one "bench" mag by removing the back part of the follower that pushes the bolt catch up. Now they all stay in a group. It may all be in my mind but it works so I dont care. Matt