Right up front, I have VERY little experience with AR-15s...a fair amount with other Stoner designs, but little with this one,

BACKGROUND

I have a 1-in-9" twist Bushmaster UltraMatch which doesn't shoot worth sour apples with light (55 gr., and below) bullets. Several "Distinguished Marksman" friends have told me that is common with 1-9 twist barrels, to try 69 gr., 75 gr., and 80 gr. bullets.

So, I want to try the 75 gr. A-Max bullets Otto Weber supplied to me.

When I try to seat them deep enough to feed through the mag, the end of the case is already up beyond the start of the bullet-nose ogive. Looks kinda bad, but looks aren't my concern. Concern is that the bulk of the bullet is actually deep back into the powder space of the cartridge case.

Usually in the rifles I am familiar with, that is a formula for excessive pressures unless loads are greatly reduced.

With bullets seated out so the start of the boat-tail is still contained in the case neck, the bullet still does not reach the end of the chamber throat, so seated-out bullets will be my first effort for determing whether this particular AR will perform with useable accuracy. Even if they shoot well seated out, that will still leave a couple of questions....

THE QUESTIONS

1. Are the Hornady 75 gr. A-Max bullets intended for use only in slow-fire, where they need not be seated deeply enough to feed through the mag?

2. If seated deep enough to feed through the mag, what is a typical powder charge weight with, say, AA-2460 (WC-846)?

ANY help/advice will be very welcome.

AC

I've been seating 75gr HPBT to 2.255 with no problems. Accurate and feeds through the mags with no problems. I should also say that 2.255 just makes it into the mag.

A 1:9 twist AR should have no problem with bullets as light as 40-45gr, as that has been my experience and several fellow AR shooters. We shoot a lot, as we have an active Highpower Club, shooting reduced targets (100yds), plus several of us who shoot the 200-600yd course.
As to what it won't stabilize, rare is a 1:9 that will stabilize the 75 AMax, or any of the 80's. That requires at least 1:8, better if 1:7, and you are right, the 75Amax is designed for single loading, hence the lousy magazine fit.
For 1:9's, the Horn 75hpbt or the Siera 77 is the best candidate for heavy mag length loading, or the reliable 69MK.
What will it shoot well? Maybe try the various 60gr choices?

I guess maybe I am trying the wrong Hornady 75 grain bullet...these are not the HP Hornadies, but the plastic-tipped A-Max's.

Whether they will stabilize we will find out tomorrow. The Hornady box does say right on it '9" twist barrels'.

And yes, that 2.255" does sound about right for potential AOL apparently. Mine will only accept 2.231" OAL but the front wall on an Orlite mag is likely thicker than on a metal one.

As to bullets 55 grainz or lighter, NOTHING of that weight has shot smaller than 2" groups at 100, with 4" more usual.

We'll see tomorrow when I go to the range if it needs these heavier bullets or if there is just plain something goofy about this particular rifle. I wouldn't be at all surprised if it was the latter, knowing from whom I bought it. It was, though, supposedly new and unfired and all factory.

Thanks to both of you for your input.

AC

I've had good luck with the 53gr Sierra flat base match bullet in rifles that don't like the 55gr fmjbt. An Armalite of mine also likes the 69gr Sierra match. My rifles all have 1 in 9" twists, I haven't tried anything heavier than the 69gr. bullet in my rifles.
A good source of info on AR type rifles can be found at AR15.com. I'm sure that this question has been dealt with before on that forum.
Good Luck,
Jeff

Thanks, Jeff -

Well the loads I tried today with the 75 gr. A-Max sure weren't the answer!! Tried 24.0 gr.s 2460 and various seating depths. None shot worth used beans. My old worn-out Garand outshot this AR today, plinking groups of about 2" with military whitebox '64 Match.

Think when I next see Otto, will ask if I can trade him back a few boxes of these A-Max for some 69 gr. Sierra HPs.

Just as a control, we also tried some 52 Gr. Sierra HPs today, and the best they shot was about 2-1/4". Using a friend's American Spirit upper today put the same 52 gr. Sierras into just over 1/2". I have previously tried moly'd 53 Sierra FB HP in this Bushmaster, and it "groups" them in about 2-to-4-inch clusters. Nothing useable there for this gun.

If the 69 gr. Sierras don't improve things, am going to junk this new Bushmaster upper and get something else....even though this is the heavy, fluted, primo stainless barrel, it just doesn't seem to even want to shoot....

Thanks for your help...

AC

I couldnt get the 75 A-Max to shoot in my 1-9 BM either. I went to the 69gr SMK.
The 62gr milsurp fmj shoot real good also.