AR brass trajectory

[dabluesguy]

OK, so I've had my M&P 15 for a couple months and have been working up loads for it. Bought some PMC 55 gr. ammo so I could break in the rifle and reuse the brass. The spent cartridges would eject to about 4-5 o'clock and 6-8 feet away (or so). On my first handload tests (55 gr. SP) the brass would end up at 1-2 o'clock about 6-8 feet away and as I increased the powder charge they would go to 2-3 o'clock, then 3-4. I am now close to the max loads on two different powders and the PMC factory loads are the only ones that land at 4-5 o'clock. I'm happy with my handloads now, both for accuracy and finding my spent brass, but I'm just wondering if the factory loads are just higher velocity or what? I don't have a chrono, so I can't tell. Don't even know why I'm posting this, cause there's no problem, but I'm just curious.

 

[Mrgunsngear]

Since the factory stuff is working as it should I'd suggest just swapping buffers when you fire your reloads; likely it will fix the issue (not really a problem IMO but I understand why you want it resolved).

 

[Palmetto-Pride]

I always find it surprising when I hear someone say they reload, but don't own a chronograph. I do own a chronograph, but I don't reload I just figured that a chrony was a necessary tool in checking hand loads. Sorry I know I totally got off the op's question I don't think you have a problem unless you can't live with it.

 

[moxie]

I believe the M&P has a brass deflector, which means that most of your brass should land at about the 2 -3 o'clock position. The 3-4:30 position would be correct without a brass deflector. The whole idea of the brass deflector is to NOT kick brass to the 3-4:30 position where your buddy will not appreciate getting hit by it.

 

[sigcurious]

That is a neat chart. For some clarification, in the 12-3 section, is m16 bolt carrier a cause or solution? I'm guessing solution as that seems to be way of the chart in general, but wanted to double check. Will definitely have to pay more attention to where my brass lands next time, all I can remember from the first firing was that it all went in a nice pile not too far away.

 

[eldorendo]

In my experience, the chart is erroneous in recommending an M-16 bc as a remedy for 12:30-1:00 o'clock ejection. I shoot several ARs, all of which eject properly EXCEPT when I use an M-16 bolt carrier, then the ejection is between 12:30 and 1:30. Swap back an AR bolt carrier and ejection's perfect, regardless of which AR.

 

[Mrgunsngear]

is m16 bolt carrier a cause or solution?

In the chart--it's a solution. However, the above poster proves all rifles are individuals and may not be the case in yours.

 

[eldorendo]

The above poster has tried the M-16 (BCM) carrier in five-six different ARs, wit the same ejection results. Doesn't bother me, as it never jams, but does eject in the 1:00 o'clock neighborhood.

 

[madcratebuilder]

The "chart" is only a guide, it's not carved in stone that the AR has to eject in the 3-5 o'clock position.

If the rifle is not functioning correct you can use the chart to help trouble shoot the rifle.

If the rifle functions correct then you can tune ejector spring pressure and extractor shape to control ejection pattern.

In my experience, the chart is erroneous in recommending an M-16 bc as a remedy for 12:30-1:00 o'clock ejection. I shoot several ARs, all of which eject properly EXCEPT when I use an M-16 bolt carrier, then the ejection is between 12:30 and 1:30. Swap back an AR bolt carrier and ejection's perfect, regardless of which AR.

Perfect example on how ejector spring pressure can influence ejection pattern. The FA bolt carrier is 11 grams heavier than the SA carrier, that is a insignificant weight difference. Try testing ejection patterns by changing carriers and using the same bolt.

 

[Jimro]

My experience with handloading for the AR is similar, a heavier charge of powder moves the ejection "clockwise." I can only assume that as the charge increases this moves the bullet down the bore faster and causes less time for gas to bleed off in the DI system, so that even though there is more gas less of it gets blown back and more out the muzzle.

An alternating hypothesis would be that the chart is wrong, that a lower charge gives lower gas pressure back to the bolt which decreases bolt velocity to the rear allowing brass to be pushed forward by the ejector button at a velocity greater than the rearward velocity of the bolt.

A third alternate hypothesis is that the chart is correct. The gas system needs to be slowed down by more mass in order to get the brass to hit the brass deflector in order to achieve "ideal ejection" or proper ejection.

I settled on a load that ejects consistently to the 2 o'clock for an across the course load and have seen no ill affects yet.

As an experiment, put a spent case in the chamber, let the bolt slam home, then pull back on the charging handle as quickly as you can. Where the case lands will tell you where your ejector button will toss brass at a very low bolt velocity.

Jimro

 

[eldorendo]

Perfect example on how ejector spring pressure can influence ejection pattern.

I'm going to give that ejector spring pressure theory a try. I'll take the bolt from the AR carrier and put it in the M-16 carrier and fire a few rounds. Will post the results back in this thread.

Thanks for the suggestion.