Light recoiling loads in the 300BLK

[300BLK]

Not wanting to further hijack Ian's "It shouldn't work" thread, I'll post here. In that thread I mentioned trying 11-12.5gr Rx7 under 203gr in a 16" carbine gas AR. All of those were full function despite H2 buffer and M16 BCG, which was a surprise to me. Holes with the 1.200" bullet were oval until the 12.5gr load. I was tempted to try the shorter 311299, but didn't have any cast. Thinking further out of the box, thanks to the Reloader 7 thread, I loaded Lee CTL-160s (170gr) over the same 11-12.5gr in Sig cases, CCI 400 primers, and crimped in the crimp groove. This latter was to accomodate my 10.5" pistol should the experiment work in the 16" carbine gas rifle. Surprise, surprise, surprise! Even the 11gr load cycled, but it took 12gr to get BHO. Too, vertical stringing was evident in the 11-11.5gr loads. 12-12.5gr were round groups, and ALL holes were round. There was some unburned powder in the barrel , but not so much to be considered problematic. Felt recoil was very soft compared to 160s at 1900fps+.

I could try the Lee 155s (160gr) and RCBS 7.26-130SPL (140gr), but doubt that even Rx7 will be flexible enough to get that done. The lighter bullet would be preferable, but WHAT POWDER could possibly burn cleanly enough and still supply enough gas/pressure to function the carbine length gas? It would have to be relatively slow, but burn cleanly. I'm thinking 2400 too fast, so leaning towards 4227 or 5744, but don't have much confidence in either getting the deed done.

 

[Ian]

5744 is out. 4227 is slower than RX7 in the BLK. AA1680 burns like wet black pepper unless the pressure is up. Try Lil'Gun or H110/WW296.

 

[300BLK]

I guess that I can't edit the title.

I have H110 and the LGS has LilGun. I've never tried the latter, and not tried light loads with H110. Do you have a suspicion or educated guess as to which would be more likely to work with light loads under the 140gr?

 

[Ian]

Too close to call. Try what you have on hand first.

 

[Rick]

Reduced H-110 loads? I've tried that in a few revolver cartridges and the result was the same with all of them. Lucky to hit a barn if you were standing inside it with all the doors and windows shut. H-110 burns poorly and inconsistent with anything less than at least close to full charges.
.

 

[Ian]

Yes indeed, but how does your 300 Blackout shoot with H110, Rick?

 

[Brad]

I guess that I can't edit the title.


But I can.......

 

[Ian]

OK, I ran some numbers for you to ponder. Assuming a COAL with the Lee 160 (@170 grains) is 2.020", the 12.0 grain load of RX-7 should generate a port pressure of around 5200 psi in a carbine-length gas system (@ ~ 6.5" of actual distance from loaded bullet base to gas port), so we can use that as a base line for minimum cycling pressure. Of course that doesn't account for gas volume, which is the real deal, but if the pressure isn't there the volume won't matter anyway. MV would be 1,127 fps and peak pressure right at 11,000 psi. Burn percentage in a 16" bbl = only 71%....but it works.

Now, let's play with H-110 and the same Lee bullet: @ 10.5 grains, MV = 1,168 fps, peak pressure of ~12,079 psi, 84% burn, and a port pressure of ~5,500 psi. That might just cycle.

Now, let's switch to the lighter bullet and try to keep port pressure at 5,500 or above with with H-110: Looks like the RCBS spire point at 140 grains and 2.020" COAL will need 11.5 grains to make 5,600 psi at the port in your carbine, which may or may not cycle depending on duration and volume of the pulse. 12,771 psi and 1,314 fps at the muzzle. 80% burn and 58% case fill.

Like Rick pointed out it may not be consistent enough at such pitifully low pressure to hit a barn from the inside, but AA1680 is the same about not burning well at all and giving horrible SDs at anything approaching reduced loads, but you may have heard of one or two (hundred thousand!) people using it to good effect for reduced loads in the 300 BLK with small RIFLE primers. You decide if the H-110 safe enough to try the reduced load with the RCBS 140 grainer. Might start a lot higher with the charge and work down?

 

[fiver]

I'd spend a little time looking at some other powders.
mp-300 or possibly something in that area might be of some use here.

h-110 in this case is too close to a case full to reduce a whole lot, I think it would be like cutting it down in a 357 or 44 mag.

 

[300BLK]

Some guys are successfully using H110 at low densities in cases like the 30-30. Speer #10 lists 16-18gr H110 under 110,130 and 150gr jacketed in a 30-30 Contender. While I have some MP300, it appears even slower than H110 in the 300BLK.

Anecdotal data from some other sources are using less than 8.5gr H110 under 230-240gr cast bullets, and even less LilGun. One of those sources lists around 10gr H110 with cast 145s from a short pistol barrel AR.

Ian,
Thanks for taking the time to run the numbers!

 

[Ian]

It's just theoretical numbers, they don't mean a thing until you put them to use. I think that around 180 grains is going to be the lightest bullet that will RELIABLY cycle a carbine system and remain subsonic, and that's assuming a gas port drilled to the max and some action/buffer tube work. My new upper wouldn't fully lock the bolt all the way back with 10.0 RX-7 and the Lee 230 powder coated, then I realized the buffer tube looked like the inside of a culvert. The lower I put it on was one of the first ones I ever built and I used bargain/sale parts for a lot of it. After removing the buffer tube and smoothing up the inside with a brake cylinder hone and ATF (removed about 15% of the total anodized coating, leaving some bare metal on the highest spots), the thing cycles the same load flawlessly with a .106" carbine gas port and my suppressor mounted up. Yay for not having to drill out the gas port! My other carbine-length upper has a bad burr from me drilling it out Bobba Fett-style.

 

[300BLK]

I'm not worried about being subsonic as much as I'm interested in a soft recoil impulse. The 170s over 12gr Rx7 seem soft, and would equate to a 191.6 power factor. The 140s at 1314 would be even softer at 184 PF. The difference between them might not even be noticeable. Achieving a really soft shooting load would be easier with pistol length gas, lighter BCG, and lighter spring/buffer, but I really don't want to spend a lot of $$ if I don't have to. All in all, its just another experiment to see how flexible the cartridge can be.

 

[fiver]

I have used H-110 in low density loads too.
the 30-06 and 19grs of 110 under a 165gr cast boolit shot okay, more 1680 shot better, and more 2230 shot even better.
jamming the boolit in the rifling to insure it didn't move from the primers blast and stop moving again made me feel a whole lot better about using it.
unfortunately your trying to move mass with gas volume which means more volume and pressure is needed to make that happen.
depending on where your hole is, is going to influence the ability to do that [or not]

since your looking for recoil 'feel' a slower launch generally provides that feeling.
[shrug] hence the 300 recommendation.

 

[300BLK]

Fiver,
Point taken regarding bullet moving. I'm not sure that the taper crimp is enough to prevent bullet movement from primer blast in this tiny case. Too, I'm wanting this load to be adaptable to at least (3) chambers, and the throats vary, so would have to load to suit the short one.

 

[300BLK]

I loaded the remainder of the primed cases with Reloader 7, 4227, and 5744. ALL were 13gr charges, but I didn't have much confidence that any of them would cycle with the 140gr RCBS 7.62-130SPL. CTL-160s (170gr) were loaded over 5744 and 4227.

13.0 5744 170gr CTL-160 left some unburned powder but cycled and bolt held open
13.0 4227 170gr CTL-160 appeared very clean burning, cycled, and bolt held open

13.0 5744 140gr RCBS very dirty, but cycled and held bolt open
13.0 4227 140gr RCBS pretty clean, cycled, and bolt held open

13.0 Rx7 140gr RCBS, some unburned powder, cycled and bolt held open

I had shot Lee 155s over 15gr 4227, and knew that they worked just fine. Dropping bullet weight 20gr and powder charge 2gr and still having a pretty clean burn and BHO was a pleasant surprise. The 13gr Rx7 load with the 140 was also a surprise as I really didn't think it would give full function. 5744 was a disappointment in that it has a relatively high NG content, and should be between 4227 and Rx7 in burn rate. Obviously it didn't make enough pressure to burn cleanly.

Reloader 7 may not yield the highest velocities in the 300BLK due to its bulk, but its clean burning at low pressure makes it very interesting.

 

[fiver]

I have taken 4227 down to 10 grs for sub sonic with a 200 gr bullet.
this was in a pistol length tube but it would be pretty close in the longer carbine tube.
I know I could lower the load from 10 in that pistol length rifle.

 

[300BLK]

Sig headstamped brass has been sized and in the process of loading. I'm going to proceed with the RCBS 140gr and 4227 at this point. I do have a partial container of AA#9 that I could try. The LGS has a 4# jug of LilGun, but I rejected that in favor of a 5# Reloader 7 that is more useful to me. IF I were simply loading for pistol length gas, this would be simpler. I might have to seriously consider a 16" pistol gas upper.

 

[Ian]

Pistol gas with an adjustable block or gas key (I have experience with adjustable blocks, but not the key) is the way to go unless you suppress it, even then it's a good system but much dirtier, gas facier, and bangs the rifle around a lot more.

 

[freebullet]

A bit off topic but, I was surprised that imr4227 provided functioning of a full size ak rifle down to 13grs under a 160harris.

 

[fiver]

our main load is 9.5 grs of AA#9 it pokes a 165 out near 1500 fps.
nice pleasant load [bolt guns remember]
I don't know how it would work in an AR.