I am working up for a AR-308 build. I plan to shoot 284 Win. Before I order the reamer.

In getting mags to test I find the PMAG 10 LR/SR has a post cast to the inside that hits the 284 cartridge. The SR and DPMS mags hold the cartridge, but seem tough on extraction. This is by pushing out with my thumb. My have to drop back to a 7-08.

Does anyone shoot the 284 Win in a semi rifle? If so what mag do you like best?

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284 win can be a pain in magazines. check for OAL clearance before proceeding.

A buddy of mine had a 6.5x284 Norma barrel custom made for him for an ArmaLite Inc. AR10B rifle receiver set, using the steel B-model mags (M14 type).  I think he had to do some tweaks to the mags to get it to feed properly.

Try ASC mags.

They allow a longer COAL than the pmag.

My ASC mag will allow a 2.867" round in the one I bought last week.

You should be able to tweak the feed lips some to feed the 284 as well.

The Winchester 100 semi-auto hunting rifle from the late '50s to early '60s came in a number of short action calibers including .284.   I had one in .308.  A buddy had one in .284.  They are very rare and collectable now.  

Anyway, the .284 had to use special magazines just for it with different feed lips because of the very issues you are identifying.  Maybe careful study of the design of the feed lips from photos of those Win 100 detachable .284 magazines would give you some clues on how to modify your mags to make them work.

The .284 was a great round and has proven its worth by subsequent wildcat modification to the 6.5-284 and other variants.

Here is a photo of an original Model 100 .284 magazine.  Notice how wide and how far back the lips are.  I also note from your photo that you are probably going to have to seat the bullets quite a bit deeper, forcing use of lighter bullets or give up quite a bit of case powder capacity.  

FWIW, that's why many of us that wanted higher velocity flatter shooting 7mm center fire rifles from that era (I'm 65), went with the long action .280 Remington, where we did not have to worry about magazine length or having to compromise on bullet weight or shape. I'm still a .280 fan.  Sorry for the diversion.  I hope the photo helps.  I admire your persistence.  A 7-08 would certainly be a lot easier.

Thanks for the help. I think the ASC mag will work with a little work. I tried the mags I have pushing the rounds forward into my barrel extension. The Pmag seems to carry the tip a little higher into the ramps but is limited to a coal of 2.820. the DPMS and ASC carry the tip lower but as suggested with a little work they can work. their coal is 2.850 on the safe side.

I only have access to 500 yrds but would like to plan on a 1000 if the chance comes up. In this I think a bullet 150gr-+ would be best. The samples I have to try were very long and would kill power space. I found the Speer 160 sp with a BC-519 to be short enough to fit the coal and not cut into powder space. The other bullets will have to wait for a bolt gun in 7mm.

My findings

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and

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Thanks again for the input/data

I've had ok luck with a Speer Grand slam 145g. It's got a bit of a flat tip and a flat base, so its 1.13"

Good luck the the ASC mags. I couldn't get them to work with factory .308 rounds.

Have you tried the M118LR PMAGs?

Britt-dog: the 145gr Speer GS has a BC of 0.327. What I have read I want a BC of at least 0.5. When I found the Speer 160 I scaled it from a web photo and was off by 0.030 in its length. If anyone has any of the Sierra bullets in the 140>160 gr range could they supply their over all length?

I think the M118LR PMAG is what I have pictured in my first post. It will work but has a COAL of only 2.250. This would keep the bullets in the 120gr range.