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Old February 7, 2016, 10:42 PM   #1
raimius
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.300 Blackout Magazine (subsonic)

For those of you who have AR-15 platforms in .300blk and shoot subsonics, what magazine works best for you?

I have noticed that Remington 220 grain loads are rather susceptible to malfunctions in my Pmags (30 round). I'm getting lots failures to feed. Sometimes the bolt does not strip the next cartridge off the top, and other times it goes nose-high into the chamber and stick. I've read that Pmags sometimes have issues with the bigger .300blk bullets, especially with the ridge about 2/3rds from the back on each side pressing into the projectile.

I do not have issues with supersonic loads, using smaller projectiles, so I assume this is a cartridge-to-magazine issue.
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Old February 7, 2016, 10:48 PM   #2
Mobuck
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I've never tried 300 in 30 round mags. P-Mag 10 round works OK as does common 20 round GI, Thermold, and Brownells.
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Old February 9, 2016, 11:33 AM   #3
Tomas
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I've had the same problems. There are two spines (for lack of a better term. I'm sure there's an 'official' name) on each side of the magazine, about 1/4 back from the front. these 'spines' are designed to keep the cartridges aligned properly, and hence, keep reliable feeding, by guiding the ammo up the magazine...5.56 ammo, that is. Your 220r bullets, and any bullet with a long shank (think 'heavy') will be interfered with by those spines. They tend to drive the bullet noses into each other causing the ammo to bind in the magazine. I've had 220s and 200gr Barnes do it. I haven't tried to alter a magazine yet to fix this, as I suspect I'd need a different follower as well, if there is one.

Best bet is to stick to bullets with a shorter shank and longer ogive as they'll be narrower at the front, and less disrupted by the spines.

I have had better luck with these types of loads using 20 round magazines, though I have no idea why...coincidence? Maybe something to do with the fact the magazine is not curved like a 30 or 40 round.

Last edited by Tomas; February 10, 2016 at 09:17 AM.
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Old February 9, 2016, 11:59 AM   #4
MarkCO
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I use stainless, teflon coated magazines with a dual wound XP spring for my 240SMK subsonics, 10s, 20s, and 30s. They run 100%. In PMags, not even close. I only run 150 grain and less in the PMags.
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Old February 9, 2016, 03:00 PM   #5
Bartholomew Roberts
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I'm using 208gr AMAX, 220gr SMK (various) and 220gr Rem in PMAGs (last generation before current gen when FDE and foilage were a color option) exclusively and have had no problems.

I've read reports of the divider in the PMAGs causing the bullet nose to hang up on long OALs but have not yet seen it personally.
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Old February 9, 2016, 11:08 PM   #6
raimius
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For clarification, I do believe Remington uses SMKs in their 220 load. The OAL seems to be a minor/non-issue. The cartridges fit in the mag with about 1/8th inch to spare. It seems to be the side indentations that are the issue.

I have not tried 20 round mags.
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Old February 10, 2016, 11:32 AM   #7
Bartholomew Roberts
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The Remington 220gr UMC I have uses an open-tipped, flat base bullet. The profile is more triangular than an SMK as well and it has a scored copper jacket with a cannelure, which the SMK lacks.

Point of aim is pretty much the same at 100yds though.
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