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Posted: 9/30/2014 10:36:37 AM EST
Someone mentioned in another thread that changing OAL would affect how much powder you an use and stay subsonic.

I've got an 8" 300 blk barrel which wont quite run unsuppressed with factory (Rem 220gr, Gemtech subsonic) ammo. Its close but I get some rounds cycling, some failures to feed and no lockback on last round.

Powder: A1680 (I was lucky to find some) at about 11.7gr seems near max subsonic
Bullet: 190gr Sierra SMK
OAL: currently just shy of mag length at 2.210"
Primers: CCI #400 small rifle
Brass: mixed Gemtech and Hornady Whisper headstamps.

So, would reducing or lengthening my OAL help or hurt my ability to tune a load for more gas while staying subsonic?
Link Posted: 9/30/2014 10:45:37 AM EST
Try applying a light crimp. Also what weight buffer are you using and what size is your gas port?
Link Posted: 9/30/2014 10:47:09 AM EST
[Last Edit: 9/30/2014 11:00:29 AM EST by delirious1]
This is just my opinion. while changing OAL will increase or decrease pressure, it's not measurable (by you). Not reliable and I wouldn't do it. I would keep OAL as recommended by the powder company. In your case, if it's not cycling right, figure out why. Could be buffer and spring. I've scratched by head as to why people change oal and don't go with what is recommended. I feel that the people who make up the charts know what they are doing and figure OAL, powder charger and bullet weight and length for a reason. a barnes 110 grain seated the same length as a 220 grain SMK will be drastically different in internal volume.

so what's it doing? Did you try other ammo (you said you tried factory ammo)?
Link Posted: 9/30/2014 11:43:42 AM EST
Gasport is close to .120, buffer is a normal carbine (not an H), and carrier is a full-auto.

There are no published loads for the 190gr sierra and A1680 to get a 'factory' OAL, as far as I know. I chose an OAL that allows the neck to grip the bullet well and also be less than mag len.
Link Posted: 9/30/2014 11:55:23 AM EST
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Originally Posted By Chrome308:
Gasport is close to .120, buffer is a normal carbine (not an H), and carrier is a full-auto.

There are no published loads for the 190gr sierra and A1680 to get a 'factory' OAL, as far as I know. I chose an OAL that allows the neck to grip the bullet well and also be less than mag len.
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Put an adjustable gas block on and your problems will be solved. You can turn the gas up or down to get reliable feeding and lock-back.
Link Posted: 9/30/2014 12:00:27 PM EST
[Last Edit: 9/30/2014 1:48:57 PM EST by delirious1]
It sounds like the buffer needs to be lighter. You are not cycling the bolt back fully and getting jams and when the last bullet fires, it's not sending the bolt back far enough to engage the catch. Make sure the BCG is smooth and lubed. I'd also experiment with what 1680 powder allows fully cycling with no problems and worth from there. I know you need to be subsonic. Try going to a 220 grain bullet which should add more pressure. Although, I've shot some 187 subsonics just fine and my 10.3 inch barrel cycles fine. I"m also using standard buffer and spring. I guess you have to find what works best with your rifle. I don't know of trimming the spring would work...I've never tried. I'd read up on that. I think most adjust buffer weight if you have your hear set on a particular bullet.

Link Posted: 9/30/2014 12:28:51 PM EST
Im interested only on the effects of OAL and powder quantity on velocity and gas pressure through the port.

Thanks guys, but Im not needing advice on gas blocks, buffer weights or port sizes, I have a pretty good understanding on those. No an adjustable gas block will not help by itself, as the rifle is already wide open, an adjustable gas block will only reduce gas unless the port is opened up as well. Yes I could certainly drill out the gas port more or lower reciprocting mass, but this thread is only about how changing the load formula might affect gas, not solving this individual rifle.
Link Posted: 9/30/2014 12:33:01 PM EST
Your gas port is bigger than mine and I've run 170s subsonic reliably but they were cast and sized to .310 and I was running IMR4227... so I can't really compare loads. With a gas port at 0.120" you can't really go much bigger. I would first add a crimp to the bullets then move to heavier bullets. 220s are a great choice.
Link Posted: 9/30/2014 12:45:31 PM EST
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Originally Posted By TXN_Infidel:


Put an adjustable gas block on and your problems will be solved. You can turn the gas up or down to get reliable feeding and lock-back.
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Originally Posted By TXN_Infidel:
Originally Posted By Chrome308:
Gasport is close to .120, buffer is a normal carbine (not an H), and carrier is a full-auto.

There are no published loads for the 190gr sierra and A1680 to get a 'factory' OAL, as far as I know. I chose an OAL that allows the neck to grip the bullet well and also be less than mag len.


Put an adjustable gas block on and your problems will be solved. You can turn the gas up or down to get reliable feeding and lock-back.


.300blk was designed to not need an adjustable gas block though.
Link Posted: 9/30/2014 12:50:12 PM EST
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Originally Posted By d16man:

.300blk was designed to not need an adjustable gas block though.
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Not to mention that his gas port is big enough that the gas block would only allow him to decrease flow, not increase it.
Link Posted: 9/30/2014 3:19:12 PM EST
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Originally Posted By tzzler44:


Not to mention that his gas port is big enough that the gas block would only allow him to decrease flow, not increase it.
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Originally Posted By tzzler44:
Originally Posted By d16man:

.300blk was designed to not need an adjustable gas block though.


Not to mention that his gas port is big enough that the gas block would only allow him to decrease flow, not increase it.


Missed that, you're right.
Link Posted: 9/30/2014 3:26:26 PM EST
Originally Posted By Chrome308:
Someone mentioned in another thread that changing OAL would affect how much powder you an use and stay subsonic.

I've got an 8" 300 blk barrel which wont quite run unsuppressed with factory (Rem 220gr, Gemtech subsonic) ammo. Its close but I get some rounds cycling, some failures to feed and no lockback on last round.

Powder: A1680 (I was lucky to find some) at about 11.7gr seems near max subsonic
Bullet: 190gr Sierra SMK
OAL: currently just shy of mag length at 2.210"
Primers: CCI #400 small rifle
Brass: mixed Gemtech and Hornady Whisper headstamps.

So, would reducing or lengthening my OAL help or hurt my ability to tune a load for more gas while staying subsonic?
View Quote


Try using a faster powder, I run AA#9 on a 16in pistol port barrel and can get 150 grain bullets to cycle.
Link Posted: 9/30/2014 3:37:06 PM EST
[Last Edit: 9/30/2014 3:45:09 PM EST by astrafire]
I had the same problem as you. I was going to get a light spring from wolff. However, I saw a NiB BCG from AIM lying on my bench and decided to try it first. Have not had a problem since.

ETA: My gun has 8 inch barrel and sometimes would not cycle 190 SMKs until I switched to the Aim NiB BCG. My 190 load is my softest shooting and most quiet suppressed cartridge, as compared to the gemtech 208.
Link Posted: 9/30/2014 4:09:17 PM EST
My first BLK rifle was the same way. I bought the upper from a new-ish vender. It would not run subs, even with a can.

I sent it back to them and they opened up the port. Still would not run subs.

I bought a Nemo custom BLK spring, it helped, but was so light it caused problems closing the bolt. I also bought an ultra LW bolt carrier.

In the end it will not run reliably enough for me. It sits in the back of the safe.

I bought a factory AAC 9 inch upper for my pistol, runs perfect. Factory AAC carbine, runs perfect. Of course, my Liberty Leo runs perfect.
Link Posted: 9/30/2014 4:28:31 PM EST
This one runs great supressed, and Ill probably be able to tweak the load with my current gas port, bolt carrier, spring, buffer, 190gr bullet, and OAL to run subsonic without the can, but the window is narrow between supersonic on the high end and reliable cycling and bolt lock back on the low end.

Just looking to see if OAL (or crimp as suggested) is worth playing with at $0.45 a round to gain any extra gas through the cycling system and improve reliability.

As things currently stand, I'd be tempted to put one supersonic round at the end of every magazine to ensure bolt lockback on the last round.
Link Posted: 9/30/2014 4:54:41 PM EST
Might be worth picking up a pound of a different powder that is indicated for subsonics to see what happens
Link Posted: 9/30/2014 6:40:41 PM EST
1680 is used to develop max velocity in that cartridge. Faster burning powder like #9, H110, W296 would be better for a 190gr sub.


Link Posted: 9/30/2014 7:07:13 PM EST
H110 seems to vary in velocity with light loads...may not meet your needs, be careful
Link Posted: 9/30/2014 9:03:47 PM EST
Dang I thought Accurate 1680 was the go-to subsonic powder for 300 Blk. I may have some more homework to do.
Link Posted: 9/30/2014 9:10:17 PM EST
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Originally Posted By rover15:
1680 is used to develop max velocity in that cartridge. Faster burning powder like #9, H110, W296 would be better for a 190gr sub.


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uh that would be a no. H110 and 296 are used to develop max velocity and 1680 is the go to subsonic powder.
Link Posted: 9/30/2014 9:10:35 PM EST
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Originally Posted By Chrome308:
Dang I thought Accurate 1680 was the go-to subsonic powder for 300 Blk. I may have some more homework to do.
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it is. ignore the above post.
Link Posted: 9/30/2014 9:11:10 PM EST
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Originally Posted By StromBusa:
H110 seems to vary in velocity with light loads...may not meet your needs, be careful
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that has been my experience trying to use it for subs. it is great for supers though
Link Posted: 9/30/2014 10:13:27 PM EST
Hornady's 9th disagrees with this completely. They list 1680 for top velocities and best to cycle subs.

I did a lot of google searches last year looking for the 190 bullet ability to cycle subsonic. I just couldn't find anyone that posted that it was a good combination. I stuck with 210+


Discussion ForumsJump to Quoted PostQuote History
Originally Posted By garyd:


uh that would be a no. H110 and 296 are used to develop max velocity and 1680 is the go to subsonic powder.
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Originally Posted By garyd:
Originally Posted By rover15:
1680 is used to develop max velocity in that cartridge. Faster burning powder like #9, H110, W296 would be better for a 190gr sub.




uh that would be a no. H110 and 296 are used to develop max velocity and 1680 is the go to subsonic powder.

Link Posted: 10/1/2014 10:44:04 AM EST
I was browsing the 300blk forum in the reloading section and a guy asked about using magnum small rifle primers. It was suggested the cut back powder by 5% and rework all his loads. Might be an option as well. They seem to think it helps to ignite ball powder better.
Link Posted: 10/1/2014 6:21:37 PM EST
^^ LOL ok. Happen to have 5 variations of the cartridge including a 300 Blackout but by all means carry on. Reloading is always a good learning experience. To the OP 8.5gr of H110 under a 180gr SP will cycle and lock my bolt back and the gas port is .102". H110 is finicky and has velocity variation like stated above which is one of the reasons I use N110 in place of it. You would probably have better success if you use a 220gr bullet.

Link Posted: 10/2/2014 5:09:38 AM EST
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Originally Posted By rover15:
^^ LOL ok. Happen to have 5 variations of the cartridge including a 300 Blackout but by all means carry on. Reloading is always a good learning experience. To the OP 8.5gr of H110 under a 180gr SP will cycle and lock my bolt back and the gas port is .102". H110 is finicky and has velocity variation like stated above which is one of the reasons I use N110 in place of it. You would probably have better success if you use a 220gr bullet.

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not sure of your post. it will or will not help to use a magnum primer?

I can say I have never needed to do use a magnum primer, but after trying a few other powders I just stick with 1680 when I can get some.

you don't mention your barrel configuration. length? gas port location? also recoil spring and buffer? I have found this to be just as important as gas port size.

Link Posted: 10/2/2014 9:03:25 AM EST
[Last Edit: 10/2/2014 9:46:08 AM EST by delirious1]
AAC designed the blackout around the remington 7.5 primer. A benchrest primer but considered a magnum primer. http://300aacblackout.com/ very bottom of page for subsonic recipe.

Their disclaimer: "For subsonic, there are special requirements and nearly all published loads are non-optimal for the 300 BLK upper. Many existing loads were developed for Thompson Center® single-shot pistols, or for ARs by people not aware of the magazine limitations. These loads pay no attention to automatic rifle function and should not be used. Look for a load which results in a cyclic rate of 700 rpm or more. A1680 is recommended because its bulk density matches the case capacity and it will generate enough gas pressure to cycle the weapon."

Primer comparison chart

http://www.handloads.com/misc/primers.asp




a nice little read on primers. http://riflemansjournal.blogspot.com/2009/06/primers-small-rifle-primer-study.html

It's easy to come with (LOLs) but I prefer real data.
Link Posted: 10/2/2014 8:53:12 PM EST
Well I think it helps to ignite a large ball powder charge but have never used a magnum primer in my sub or super loads. As far as what I have barrel length varies from 8" to 16.5" in 300/221, 300 Fireball, 300 Whisper (sold) and 300 BLK. My reamer and gauges are for 300/221 so that is what I make myself. Gas port (.095 - .102") is usually in pistol position and a standard carbine buffer and spring is used. Not a lot of tweaking is needed if you use a 220gr - 240gr bullet.



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