Hey all, I’m trying to put together a .300 BLK with a 7.5” barrel. PSA has some great deals on uppers, but they are all in the 1/8 twist. I’d prefer the 1/7. I’ll be shooting supers, will this work with the heavier loadings? Or should I hold out for the 1/7? Thanks

it's a 7 1/2 inch barrel...how far are you gonna shoot it? and does that 1:8 twist really make a difference at those distances??

What bullet weights, types of bullets, and velocities do you expect? There are ways of calculating what twist rate is necessary for bullet stability, but that information is necessary. As I recall, the SIG 9" MCX barrel has a 1/5" twist rate, so there seems to be some purpose in faster twists for the cartridge from short barrels, although probably for heavy subsonic loads. I have a Thompson/Center Contender 1/8" barrel, and I have seen instability in bullets heavier than 190 grains. The supersonic loads I am aware of, though, are typically much lighter and seem to be fine from 1/8, but that’s also from a ~17 inch barrel.

For maximum precision, slower rifling twists are usually better, but I doubt it would be significant with the type of gun you’re considering, and there’s not much difference between 1/7 and 1/8.

Distances here probably won’t exceed 50 yards. As far a bullet weights, about all you can find are the 220 gr OTM’s.

quote:

Originally posted by bigarms:
I’ll be shooting supers, will this work with the heavier loadings?

So will you be shooting supersonic loads with bullets in the 110 to 150 grain ballpark, or subsonic loads with bullets in the 188 to 220 grain ballpark? If both, which ones do you expect to shoot more often?

What's your primary use -- plinking, hunting, HD, paper targets, steel targets?

Right around the 150 gr bullet weight, you begin to get competition between powder volume and bullet in case volume. Higher grain bullets typically will not be able to be loaded to higher velocities simply because you won't be able to fit enough powder in the case without exceeding pressure limits. These are just general guidelines, not absolutes.

From what I've read on 300BLK specific forums most high velocity loads are achieved using 135 gr and under bullets. Heavier bullets are generally loaded to subsonic speeds. It's just the nature of the cartridge.

If you load your own you will come to this conclusion through trial and error as the character of this cartridge becomes apparent to you. 1-8 works great for supers and subs. Both of my guns are sub MOA at 100 yards. One is for supers (16") and one is for subs (10"). YMMV

My 8.5" PSA .300 upper (unsupressed) will not cycle heavy rounds well at all. I stick with 150 grain and less and haven't had issues.

At the moment, I’m not set up to load my own .300. And most available factory ammo here is the 220 gr supers. This will be used mainly as a range gun, but if pressed I’d prefer it be able to handle an HD role as well. Just trying to achieve the best performance over the widest selection of ammo loadings. Again, thanks for the responses.

Are you located someplace that doesn’t allow you to order ammunition online? That would be the only reason to rely on one specific load.

But if so, I’m curious about which that specific load is. I didn’t research every possibility, but to cite one example of a source, MidwayUSA that carries a large variety of 300 Blackout ammunition does not list a supersonic 220 grain bullet load (that I could find).

And to return to my experiences with a 1/8" rifling twist barrel, I have had definite stability problems with 220 grain loads; not with every shot, but with some. That may not matter much at 50 yards if you’re not using a suppressor, but given a choice, I would choose the 1/7 over the 1/8.

quote:

Originally posted by bigarms:
And most available factory ammo here is the 220 gr supers.

I can guarantee for a 300 blackout you mean 220 subsonic loads. Well, unless you live in a climate where your speed of sound is less than 1,000 fps.

I have an Wilson Combat 11.3" blk upper with 1/8 twist. All rounds through the upper have been suppressed. I've shot subsonic loads with 190-208 grain bullets to 100 yards, with the 190 and 208 grains to 150 yards. All subsonic rounds have exhibited stable flight. No key holing in paper or steel, and recovered bullets show obvious signs of nose-in-first impacts on steel and dirt. But in comparison to all my other rifles, the subsonic rounds drop like a frickin' brick and aren't all that accurate.

Subsonic loads I've tried -- Hornady 190 Sub-X, S&B 200 FMJ, Hornady 208 Amax. MVs of 1,100 to 1,140 fps.

Next are the mid-weight FMJ supersonic loads -- American Eagle 150, Magtech 123, ADI 144 MSR, Fiocchi 150. I find the AE and Magtech loads OK-ish in accuracy, along the lines of better FMJ 55 grain 223 loads. The ADI and Fiocchi loads were....bad. MVs around 1900 fps for the heavier FMJ, but the Magtech 123 is 2,182 fps.

The fasted supersonic loads are the 110-125 grains. MVs of 2150-2350 fps. And the most accurate rounds, too. Hornady's 110 VMax works, as does Vortex 110 and 120 TAC-TX. It's probably barrel dependent, but the 125 SMK bullet is generally good for me. My most accurate load is from Aussie Outback, which I've shot out to 370 yards so far.

Understand that FMJ loads don't appear to tumble or fragment at blackout velocities. I sure wouldn't use them for HD. The SMK bullets mushroom beautifully against AR500 steel, but exhibit inconsistent expansion when dug out from dirt (i.e. when I miss the steel target). The 110 Vmax expands really well in both soft and firm dirt. The TAC-TX bullets expand at the nose area, but the rear portion of the bullet remains intact.

For subsonic loads, the Amax does expand a bit at the nose. The Sub-X does a much better job of nose expansion in dirt -- with nice copper petals peeling backwards.

As Sigfreund states, order your ammo on-line, unless laws prohibit this.

As for subsonic loads (heavy bullet, moving below the speed of sound), I'm not all that thrilled with them. The ballistics just seem way inferior to me. I'll probably keep the S&B ammo in stock because it's cheap and will allow my to practice AR-15 work on close steel without denting my targets.

As for supersonic loads (mid-weight bullet, moving above the speed of sound), I will probably keep two loads -- Hornady 110 VMax and Aussie Outback 125 SMK. Both of these are reasonably accurate out to a few hundred yards. For home defense I'd probably use the 110 Vmax, solely based on the bullets I've dug out from the ground.

If I hunted game that was a reasonable size for 300 blackout, probably the Barnes TAC-TX loads would be the ticket.

1/8 twist works for me, with a 11.3" barrel. YMMV

Well holy shit! So, after putting on my glasses I “see” the 20 boxes of .300 I purchased are in fact 120 gr. So, I’m assuming the 1/8 will handle that. Still I may hold out for the 1/7. Sorry for my obvious lack of visual acuity in describing my dilemma. Thanks to all for the replies.

I’m still curious about what specific ammunition you’re referring to. But even if 1/8 might be marginal for 220, there’s no reason why it wouldn’t stabilize 120 with no problem.

For anyone who may read my posts and wonder about the stabilization problems I’ve had with a 1/8" barrel, my tests and evaluations were for the purpose of determining the safety of using my suppressor with various loads. The variables among different ammunition and bullets, and even suppressors, are things to consider. My suppressor is rated for .30 caliber magnum cartridges and is longer (9") than some I’ve seen. It’s also possible that the end cap hole sizes vary and what might cause a baffle strike in one suppressor may be fine in another.

My tests involved shooting at card stock from as close a distance as possible without destroying the paper with the muzzle blast. When I see evidence of bullet yaw (and that’s easy to detect when using clean card stock), then I become concerned. Bryan Litz points out that bullet yaw is greatest at the beginning of the flight (discounting, of course, what happens at long range when it becomes unstable), and inside the suppressor is obviously when we need to be most concerned.

Plus those tests are useful only to a limited degree. With one 220 grain load I was testing I fired a dozen shots without any problems only to have one bullet graze the end cap. In fact, I have come to suspect that suppressor design may be a factor. With the same 220 grain load and suppressor, some bullets showed yawing at 70 yards while those fired without the suppressor did not. Is it possible that gas blowby in a large suppressor could affect the flight of a subsonic bullet? This is a question I’ve not seen in the countless discussions about suppressors, but it seems plausible (without my knowing much about suppressor designs and effects).

As other commentators have noted, there is no good reason to use subsonic ammunition for any serious purpose, unless it’s taking out a sentry or guard dog during a black ops mission. In my limited testing, for example, the Hornady 208 grain A-MAX bullet doesn’t expand at all at subsonic velocity in soaked newsprint, water jugs, or even soft dirt. Hornady’s relatively new 190 grain Sub-X load does expand in water and (according to the Internet) in gelatin, and is reasonably accurate in my gun, so that’s what I’d use if I absolutely, positively had to use subsonic for something that mattered.

My 300 BLK features a 16 inch barrel with a 1:8 twist and I typically load 125 grain bullets using H110. When I take the time to mount a scope to it using a cheap 3-9 scope on a Burris QD mount I will typically get 3/4 MOA or a tiny bit less. BTW, I built both of my 16 inch rifles to be mainly sighted using the traditional post and aperture sights, with which I am quite pleased when I can manage 2-3 inches at 100 yards.

This message has been edited. Last edited by: Scooter123,

quote:

Originally posted by bigarms:
I “see” the 20 boxes of .300 I purchased are in fact 120 gr.

I'm guessing you have Remington UMC ammo. The UMC is relatively inexpensive, and much of the 120 grain loads have the more expensive copper hunting bullets.

Buying 20 boxes of ammo before even owning an upper isn't the path I would take. I tend to buy a couple of boxes of any one untested ammo, then shoot a box each on two different days.

Remy UMC performed OK in my upper. Nothing special -- roughly equal in accuracy to AE 150 FMJ and Magtech 123 FMJ.

The only real mistake I made in 300 blk was buying too much Sig 125 HPBT at one time. The first box performed pretty well. The following week I saw it on deep discount at Cabela's, so I grabbed the 8 or 9 boxes on the shelf. Turns out the ammo just doesn't like my rifle's chamber. I've experience 4 popped primers so far -- roughly 1 per box. Three of the popped primers got jammed on the barrel breach face, taking the gun down until I could dig them out.

The fourth popped primer also jammed the gun, but this time the primer dropped free when I dumped the mag. Unfortunately the next round became jammed in the chamber -- a live round!. For whatever reason the case was jammed pretty hard into the chamber, but yet not fully chambered. I could not extract the round with the tools I had that day at the range. Once I got home, I was able to extract the round. No more Sig 125 for my blackout. YMMV