Help please. AR shooting different groups from each feed ramp

[Luke2236]

Let me preface by saying I'm a bolt guy, somewhat new to gas guns so this may be an obvious one for you senior level folk


Noticed a new 308 jumped from a .5 gun when hand feeding to 1.3 when feeding from the mag. Shot 6 shot groups with multiple mags and alternated each shot at 2 bullseyes. Shots 1,3,5 (right feed ramp) at 1 mark, shots 2,4,6 at another.

My right feed ramp groups averaged .94 and were centered about .5 high in relation to the bullseye
My left feed ramp groups averaged .5 and were centered about .4 to the right in relation to the bullseye

I have polished the feed ramps to within an inch of their lives, zero change.

Any input would be appreciated

 

[Scottmilk9]

Load from the magazine and then take it out of the chamber. See if its dented at all. If so, this can cause different pressure and cause the erratic groups.

Can you tell if there is a good chamfer on the chamber?

 

[Glen Seekins]

That's weird.. You should shoot one round, rack next out, shoot another, rack it out.. Repeat so feeds from one side. Then do it on the other. Unless your case is getting smashed and sitting sideways in the chamber I couldn't see it change that much I wouldn't think.. Definetly look at the cases like Scott mentioned. Also try different mag. Knights SR25 mags feed way smoother than a pmag does. Also sets the case a bit higher so they don't hit feed ramps as hard.

 

[bp7178]

Poor neck tension?

 

[mtrmn]

I have a shooting buddy that reports the exact same thng with an Armalite 16" 308 carbine. POI alternates between 2 separate groups depending on which side of the mag the round feeds from. He did try different mags with same results, but that's all I know. If he has figured anything out with it he has not said anything. I'll check with him and see. I have not personally witnessed this.



UPDATE: Sorry but the buddy has not done any more troubleshooting on this problem.

 

[Luke2236]

No dents in the case at all, I've also polished the ramp and chamber.

I went by Creedmoor today to ask some of their mechanics if they had seen this before, they brought out their most knowledgeable gas gun guy and he looked at me like a pig looking at a wrist watch so either this is rare or they aren't that smart.

Neck tension could be an issue. I am going to pull out some Fed GMM and run them through to see if they do the same thing. If that cleans it up I will have the culprit as my loads are identical to FGMM except I don't crimp.

If that's not it, I'm lost.

 

[KRG762]

Research first round last round phenomenon regarding ar10 platforms. You are experiencing bolt lift from all of the rounds pushing up from the magazine differently. When you examine your rifle you will notice all ar platforms have bolt carrier groups that fit in the upper with free play. What happens is when the loaded magazine pushes up on the carrier it cause the bolt to shift the round in the chamber which results in a unstable bullet" jump" in the free bore transition into bore. Hence" first round last round". Those two rounds will always be the worst in the group. Basic destabilized resulting in larger groups. Without the mag or using a sled you will notice much tighter accuracy.

Of course gas guns are much more difficult to master then a fine tuned hand build bolt rifle.

There are excellent ar10 videos available on the hide addressing tips for running a gas gun.

Sometimes they will make you scratch your head twice.

 

[KRG762]

No dents in the case at all, I've also polished the ramp and chamber.

I went by Creedmoor today to ask some of their mechanics if they had seen this before, they brought out their most knowledgeable gas gun guy and he looked at me like a pig looking at a wrist watch so either this is rare or they aren't that smart.

Neck tension could be an issue. I am going to pull out some Fed GMM and run them through to see if they do the same thing. If that cleans it up I will have the culprit as my loads are identical to FGMM except I don't crimp.

If that's not it, I'm lost.

I have addressed your issue. This is not uncommon and is a very well known topic in this section. May I ask what rifle you are running too? Some have looser tolerances which coincide with what I originally spoke about.

 

[Cranedaddy]

Research first round last round phenomenon regarding ar10 platforms. You are experiencing bolt lift from all of the rounds pushing up from the magazine differently. When you examine your rifle you will notice all ar platforms have bolt carrier groups that fit in the upper with free play. What happens is when the loaded magazine pushes up on the carrier it cause the bolt to shift the round in the chamber which results in a unstable bullet" jump" in the free bore transition into bore. Hence" first round last round". Those two rounds will always be the worst in the group. Basic destabilized resulting in larger groups. Without the mag or using a sled you will notice much tighter accuracy.

I was just having a conversation with a guy at my local range about this. We were kind of joking about the gun throwing the last round. He said he didn't know what to do, other than load 6 rounds to shoot a 5 round group.

 

[KRG762]

You got it. Every round will shift differently. Think about each round pushes up from the magazine onto the carrier with different pressure as the magazine unloads. For what ever reason ar10's suffer from this more than 15's.
I gotten 5 shot groups down in the 1/4 moa range at 100yds with my hand loads out of ar15's. Ar10's loading from the mag typically 1" or better and you've got a good rifle. .5" even better!

Gas guns are meant for whacking bad guys or critters quickly. And they are flat out fun to pound steel with.

az's eCranedaddy;3140959]I was just having a conversation with a guy at my local range about this. We were kind of joking about the gun throwing the last round. He said he didn't know what to do, other than load 6 rounds to shoot a 5 round group.[/QUOTE]

 

[Luke2236]

You got it. Every round will shift differently. Think about each round pushes up from the magazine onto the carrier with different pressure as the magazine unloads. For what ever reason ar10's suffer from this more than 15's.
I gotten 5 shot groups down in the 1/4 moa range at 100yds with my hand loads out of ar15's. Ar10's loading from the mag typically 1" or better and you've got a good rifle. .5" even better!

Gas guns are meant for whacking bad guys or critters quickly. And they are flat out fun to pound steel with.

az's eCranedaddy;3140959]I was just having a conversation with a guy at my local range about this. We were kind of joking about the gun throwing the last round. He said he didn't know what to do, other than load 6 rounds to shoot a 5 round group.

[/QUOTE]

I hear what you are saying, researched it and it makes sense. Still not sure how it can cause the below though.

"My right feed ramp groups averaged .94 and were centered about .5 high in relation to the bullseye
My left feed ramp groups averaged .5 and were centered about .4 to the right in relation to the bullseye"

Across multiple mags, one feed ramp consistently shot smaller groups, and centered them in an entirely different area, than the other feed ramp did.

Thoughts?

 

[Tacsnipe73]

What upper are you running?

 

[rg1911]

FWIW, having heard of the "first round, last round" effect, when I am developing a load for my ARs and my bolt guns that have a magazine, I single-load. This removes at least one variable.

Richard

 

[HOGWILD]

Nice thread guys I learn something every day ! I guess I am lucky it was not involving a female or it would have cost me more $$$$ !! No offense to you female shooter's BTW it's just normal I have to learn stuff the hard way I guess that is why they have 30 round magazines as you can get them bad guys with atleast 14 rounds so you are still talking SMOA with an AR ? I would not have too much of an issue with that any day of they week but do understand you are very observant for sure!! What kinda AR ya shooting and what Magazines are ya using? Or does it matter what magazines ya use?

 

[Luke2236]

Upper- DPMS
Mags-Tried multiple, most were Pmag
Barrel- 18" Kreiger

 

[Luke2236]

The attached are some of the photos I took, left feed ramp versus right. Obviously different impact point. Consistent across mags.

 

[Tacsnipe73]

Damn... and from a kreiger too. that sucks

Who chambered your barrel?

 

[Tangodown911]

I too have experienced this lately. I wonder if switching bolts to one with a tighter tolerance would help tame this?

 

[tt123]

I really don't understand how this can be happening unless the rounds are getting damaged somehow on the way from the magazine and into the chamber. The way I see it, once they're chambered and ready to fire, it shouldn't really make any difference how they got there.

Are you shooting reloads or factory ammo? If reloads, could insufficient neck tension be the cause?