Mag Feed Accuracy

[Kritter]

I have a AR built with a Lilja ar740 in a noveske receiver set w/maxim BCG
The problem i have narrowed down is when i single feed the gun will shoot 1/2" and when it is magazine fed it shoots 1-1/2"-2" groups. I reload i use lapua and LBC brass using forster dies. best load is 28.5 xbr .010 jump with 123 amaxes (no crimp) lee crimp shot worse

Any Ideas why it shoots that bad when fed through the magazine (ar-stoner)?

i figured this out later because i was single feeding during load work-up

[Vasux86]

Check your neck tension. I've known of folks having the same problem because the bullets were sliding forward when the rifle chambered from rounds from the magazine.

You could check this measuring COAL before you put a round in the magazine, load it in the mag, let the slide come forward and chamber the round, and then remove the round and measure COAL again.

[Kritter]

Check your neck tension. I've known of folks having the same problem because the bullets were sliding forward when the rifle chambered from rounds from the magazine.

You could check this measuring COAL before you put a round in the magazine, load it in the mag, let the slide come forward and chamber the round, and then remove the round and measure COAL again.

I measured a piece of sized LBC brass and ID of neck is .264 I'm guessing too lose and i guess i would have to run bushing dies... anyone mess with a Forster sizer button to tighten them up?

[Kritter]

Any recommended dies? i am used to my lee collet kneck sizer with my .308
Would sanding my expander button on a drill help?

[LRRPF52]

I'm leaning more to Natural Point of Aim disruption from the return to battery stroke of the action when the gun feeds normally.

Are you bagging the position heavily, both front and rear?

[NugginFutz]

It's fairly easy to check if neck tension is your issue. Measure COAL of a few test rounds. Load up normally and fire a round. Do NOT fire the next round, but extract and remeasure. If the COAL has changed (+ or -), you have your answer.

If you find that neck tension is an issue, either reduce the sizer ball diameter or you can pick up a Lee FCD. With the FCD you can increase the crimp until setback or sliding is eliminated.

Several members can also tell you that using Redding bushing dies can also do the same without a sizing ball.

Good luck on ID'ing your issue.

[Vasux86]

Any recommended dies? i am used to my lee collet kneck sizer with my .308
Would sanding my expander button on a drill help?

What is the OD of the expander button? This probably isn't the issue but you can easily measure to see if the expander button is limiting resizing or the issue might be found elsewhere.

[sneaky one]

Yes , kritter we have. On the Forster button- easy to fix , here and upon most. Hone it down a bit- take good measurements as you go- don't go past .260-.259.

[Kritter]

I'm leaning more to Natural Point of Aim disruption from the return to battery stroke of the action when the gun feeds normally.

Are you bagging the position heavily, both front and rear?

i normally shoot prone off a harris bipod and rear bag (load work up) but have tried it off a lead sled with same results

[Kritter]

Thanks all I will try to do the coal test. I have a lee crimp die but didnt seem to help groups but have never checked for COAL change, I will try again. My Expander ball measures .264 i sanded it down to .263 before i read this. I think Neck tention may be why i was getting better groups with virgin brass?

[NugginFutz]

Thanks all I will try to do the coal test. I have a lee crimp die but didnt seem to help groups but have never checked for COAL change, I will try again. My Expander ball measures .264 i sanded it down to .263 before i read this. I think Neck tention may be why i was getting better groups with virgin brass?

Did you develop your load using unfired new brass? If so, be aware that case volumes will be slightly different between new and fired brass. I'd done this, and saw a drop in velocity with the same load in the fired brass. A significant change in velocity can clearly have an effect on group sizes.

[Kritter]

Did you develop your load using unfired new brass? If so, be aware that case volumes will be slightly different between new and fired brass. I'd done this, and saw a drop in velocity with the same load in the fired brass. A significant change in velocity can clearly have an effect on group sizes.

My current load was tuned with once fired brass FL sized with shoulder bumped back a few thou. Using Forster datumdial. When I put cases in a case gauge they stick out a bit so I think my chamber a slightly big but passes go/no go test

[lwminton]

It's fairly easy to check if neck tension is your issue. Measure COAL of a few test rounds. Load up normally and fire a round. Do NOT fire the next round, but extract and remeasure. If the COAL has changed (+ or -), you have your answer.

If you find that neck tension is an issue, either reduce the sizer ball diameter or you can pick up a Lee FCD. With the FCD you can increase the crimp until setback or sliding is eliminated.


Several members can also tell you that using Redding bushing dies can also do the same without a sizing ball.

Good luck on ID'ing your issue.

Does a magazine fed round enter the chamber in line with the bore? It looks to me eyeballing that it enters at a slight angle. If so I would think that could affect concentricity or grip. Just guessing.

[LRRPF52]

A cartridge enters the bore relatively centered, especially as it snaps into the bolt face recess.

It also has a lot of tension on it from the ejector and extractor, but gets aligned with the chamber. There isn't a lot of room to get off-center in there, unless the projectiles are experiencing runout from the press.

A way to avoid runout is to seat your bullets with two strokes, rotating 180 degrees after the first stroke. Large sample tests have been done with this method to demonstrate its effectiveness.

[lwminton]

A cartridge enters the bore relatively centered, especially as it snaps into the bolt face recess.

It also has a lot of tension on it from the ejector and extractor, but gets aligned with the chamber. There isn't a lot of room to get off-center in there, unless the projectiles are experiencing runout from the press.

A way to avoid runout is to seat your bullets with two strokes, rotating 180 degrees after the first stroke. Large sample tests have been done with this method to demonstrate its effectiveness.

I do the 180 twist but don't have a run out tool to check. There is a tool (Hornady?) to measure then correct run out-is it worthy?

http://bulletin.accurateshooter.com/2015...m1-garand/

Coincidently, I just watched this video of an M1 in slow motion. It shows the tilt of the bullet as it enters the chamber. It is hard to see if it completely straightens out before the lands but it looks like a close call. I guess I was thinking that the bullet grazing the top of the chamber is what oriented the bullet flat.

[Kritter]

I have finally run a tested you guys have recomended.. checked Coal (ogive) and Concentricity of 4 rounds before and after cycling through rifle and there were no real changes mabe a bump of .001 Concentricity of 1 round. I have now adjusted my Forster button so my neck tension is now .262 after FL Sizing (was .264) i will be going to the range to test groups i will report back after.

[Bill257]

The gunsmith who runs my local range tells me he sees ARs that have a slightly different points of impact based on what side of the magazine the round is fed from.

His method to determine this is shooting at two different targets - one for rounds that are fed from the left side of the mag and the other for rounds fed out of the right side. Compare these groups to groups produced when all rounds are fired at the same target or overlay the left and right targets. (for example the left and right groups could each measure an inch, but when combined, as if shot at the same target, the measurement could be 2 inches)


If the cartridges loaded with the modified expander ball don't prove to be the solution, this two target test may point you to the side with the issue. The and you could examine differences in the feed rams or magazine feed lips.

In .223 my factory crimp die slightly improved groups with some bullets and really opened up them up with another already accurate load / bullet - I'd minimize variables by only shooting one type of ammo at a time and labeling each target.

Good luck and keep us posted on your findings

[LR1955]

The gunsmith who runs my local range tells me he sees ARs that have a slightly different points of impact based on what side of the magazine the round is fed from.

His method to determine this is shooting at two different targets - one for rounds that are fed from the left side of the mag and the other for rounds fed out of the right side. Compare these groups to groups produced when all rounds are fired at the same target or overlay the left and right targets. (for example the left and right groups could each measure an inch, but when combined, as if shot at the same target, the measurement could be 2 inches)


If the cartridges loaded with the modified expander ball don't prove to be the solution, this two target test may point you to the side with the issue. The and you could examine differences in the feed rams or magazine feed lips.

In .223 my factory crimp die slightly improved groups with some bullets and really opened up them up with another already accurate load / bullet - I'd minimize variables by only shooting one type of ammo at a time and labeling each target.

Good luck and keep us posted on your findings

Bill:

It isn't which side of the magazine but rather he is shooting at one target that is left and one that is right. He is probably centered on one of them because that is where his position is focused but when he transitions, he will probably shoot to the direction of the target to the side. My bet is that if he set in one target and shot a group, pulled that target and set another one about five feet to the side of the location of his first target, he would also shoot center. This is because he would get into a new position for the second target.

Put in a bank of five targets, get a position on the center target, then shoot from right to left or left to right in rapid fire. My bet is that the far right target will shot a group to the right of center, the center target will probably be centered, and the far left will probably have a group to the left.

LR55

[LRRPF52]

Yup. I see this in dot drills all the time, as well as on an array of targets when doing accuracy testing. Even the small angle that is required to aim up a little more at the top set of dots is enough to make changes in the position that don't support good grouping.

What I commonly see is a trend where the lower targets on the paper, which are more level with the position, end up being the ones where the groups tighten up. For me, settling the gun into the position, no matter what vertical angle I'm aiming, eliminates this trend.