Weird results 7.62x39 subsonic, any ideas ?

My previous load for x39 subs was the 200gr cast lead semi wad cutters off TM, with 6.5 gr of Trail Boss. Gives a nice consistent speed just below supersonic.

Now I am trying some of the .312 Hornady 174gr Interlock RN. Started low and slowly increased the powder in .5 gr steps. Just can't seem to get it over 900fps ?!? I am up to 9.9 gr of TB, and if anything it seems to be slowing down....

Not sure if the 312 projectiles are generating too much friction or what the hell is going on, but I am a bit stumped.

Any theories or suggestions welcome.

Jacketed bullets will require much more pressure to engrave them and push them down the bore compared to a lubricated lead one. That may be your answer.

That^^^^ and safe practice with trailboss is work down from full case....perhaps lucky not to stick one in the bore:oh noes:

Yeah, with the mild loads I make a point of having a look down the tube after each shot.

Starting to get to the point where the TB loads is close to compressed. So if I keep going with these projectiles I may need to go to a different powder ?

Quote:

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Originally Posted by ebf

Yeah, with the mild loads I make a point of having a look down the tube after each shot.

Starting to get to the point where the TB loads is close to compressed. So if I keep going with these projectiles I may need to go to a different powder ?

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TB shouldn't be compressed but there is compressed and compressed
It is made into donuts specifically to take up case space with light loads.
I have always started with a full case (cant fit more in full) and compressed the neck bit when seating the projectile BUT that was on 308 capacity cases and greater.
If you are honestly concerned about compression then yes time for another powder.
Personally I would rather some light compression than light load density.

If I want to go to a different powder, do I use something faster (Red Dot) or slower (AP70) ?

Quote:

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Originally Posted by ebf

If I want to go to a different powder, do I use something faster (Red Dot) or slower (AP70) ?

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I have not used those powders and do not feel qualified to comment on them.

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Very odd but may be undersized projectile. Primer may be pushing the bullet down the bore before pressure builds. try ap70 about 10 grains will do.

Quote:

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Originally Posted by ebf

My previous load for x39 subs was the 200gr cast lead semi wad cutters off TM, with 6.5 gr of Trail Boss. Gives a nice consistent speed just below supersonic.

Now I am trying some of the .312 Hornady 174gr Interlock RN. Started low and slowly increased the powder in .5 gr steps. Just can't seem to get it over 900fps ?!? I am up to 9.9 gr of TB, and if anything it seems to be slowing down....

Not sure if the 312 projectiles are generating too much friction or what the hell is going on, but I am a bit stumped.

Any theories or suggestions welcome.

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Trail Boss is a funny powder to work with outside what it was designed for which is to perform like black powder it's pressure curve mirrors black powder's almost perfectly
more of an explosion than a fast burn, Hogdgon recommends to start with a 70% case full and go up or down to find the best charge, I've only worked with 7mm08 and .308
in rifles and cowboy loads in pistols, my findings are that Trail Boss does not give very high velocity about 1200fps with a cast 151HP and the accuracy went out the window
big time, you are trying to shift a heavy for cartridge jacketed bullet that maybe .001" oversize, it may not be a compressed charge and it is probably not high pressure
by normal standards but Trail Boss is in a class of it's own, Sounds like you have reached the limit were Trail Boss stops being useful, it's hard to find 7.62x39 load data with
heavy bullets even cast data stops at 160 grain bullets, AP70 and AP100 maybe the way to go on the slow side and AS30 and AS50 on the fast.

Ive never liked the idea of subsonic jacketed rifle bullets. Normal ones this slow will have zero expansion on target and there is always the risk of getting one stuck when doing development. Better imo to stick to lubed cast or if you must have jacketed find a pistol bullet of the right weight and diameter, they have thinner jackets and are designed to perform at low velocity.

Nothing worse than getting a jacketed bullet stuck in a barrel it's very hard to get them out, A friend of mine did when trying to make quite plinking loads was before suppressors for centerfire
rifle were available back in the dark ages, luckily a gunsmith got the bullet out without damaging the bore,
Thats what got me and a few friends into casting a lot of shooters said we were nuts at first we were but after quite a few fails we got pretty good at it and started beating them at Deerstalker
shoots,
The problem with pistol bullets is there are not very many available in .30 cal and those are to light, If one was keen on shooting cast bullets the best option is the .358 Winchester cast bullets
from 100 grains to 310 jacketed the same weight range in the heavier weights 200 plus grains the cast bullets and jacketed can be shot at the same velocities it is the most favourite cast bullet calibre
in the US everyone either has one or wants one, for inside 200 metres it will kill any animal in NZ , So you guy's get some built and I will get the moulds I need to feed my addiction to buy more moulds

ebf...stop pissing around with jacketed and buy some cast of shootersnz .....the 151s are great and if you want 170s ...well they are too,look identicle without the hoho hollow point.
you will need somewhere between 4 and 7 grns TB powder to get the speed about right...must put my loads over chrony one day......

looks like I am going back to cast projectiles, thanks for all the useful info guys, much appreciated :thumbsup: