Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

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jason867
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Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by jason867 »

Trying to develop a subsonic loading for my rem700 24" 308 win rifle.

Rainier "Wedge" 180gr plated flat nose and flat base projectile
8.7grs Trailboss powder
CCI #200 or Fed #215 magnums

8.7grs got me to an average 1050fps with CCI #200s, switching to magnum primers added about 20fps.

Through all of my testing I've noticed the following.

The first shot in shot-strings would almost always chrono slower, between 850-950fps, with followup shots averaging closely and consistently around 1050fps.

Then I noticed, this always happened whenever I left the action open during reloading or whatever.

So I tested that. I shot five shots in a row, allowing a minute or so of barrel cooling/airing, and the velocities were all lower.

Then I rapid fired several shots, not allowing any time for the chamber to remain open, and velocities were very consistent around 1050fps.

This showed up on paper as well, the slow shots would impact much lower.

I would think allowing the barrel to air out would cause increased velocities if anything, because of the increased oxygen present. But for some reason it's doing the reverse!

Can anyone explain this? I'm probably going to try Titegroup powder next, because I can't seem to get Trailboss to stop this behaviour.
johndoe3
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Re: Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by johndoe3 »

We've discussed this particular problem before--you're not doing anything wrong. The problem is caused by the barrel length--increased friction (there is a research paper on this by a maker of subsonic ammo).

If you had an 18" or 16" barrel, you would see a much smaller (negligible) difference in velocity between the 1st subsonic shot and subsequent shots.

The same thing is seen in 22LR with a 16"/18" barrel versus actual lower velocities in a 24" barrel in those old lever action 22LR rifles--the extra barrel length is just for show it adds nothing to performance, except longer sight length with using iron sights.

Go ahead and try Titegroup or other substitutes, and you'll see that the same issue happens with those other powders.

So...you'll just have to live with it in shooting subsonic 308 with that rifle with the 24" barrel.
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jason867
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Re: Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by jason867 »

Anyone have a link to that research paper?
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T-Rex
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Re: Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by T-Rex »

I attribute this more to temp, than anything else.
The first shot warms the chamber, which will warm the next case and powder. Warmer ammunition will always have a higher velocity. This is why rounds developed in Summer/Winter will have different velocities when tested in the opposite season.

If you wanted to blame friction, in this particular case, wouldn't it be in reverse?
The warmer, dirtier barrel would cause subsequent shots to be of lower velocity, no?
I say this because heat will expand the barrel (albeit minimal) as well as the fouling reducing friction. Less friction = lower velocity.

I went through all of this while developing loads for my 20" 308, with 1:10 twist.
I use (IIRC) 10.0gr Trailboss for 180g RN Sierra 2170.
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jason867
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Re: Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by jason867 »

I'm not 100% convinced it's temperature related. I did most of my load development through the winter here in Indiana, and these subsonic loads don't produce much heat anyways. And I wasn't blasting them through my Barrel, I was slowly measuring each one over a chronograph.

I just wouldn't think the barrel would heat up enough in these conditions to throw off the velocity that much.
jason867
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Re: Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by jason867 »

Unless, possibly, it is just the nature of subsonic rounds to be more susceptible to these variables.
jason867
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Re: Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by jason867 »

I just came across this article regarding subsonic ammo fired in Long barrels.

https://ebrammo.com/info-tools/subsonic ... ng-barrels
Tackleberry47
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Re: Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by Tackleberry47 »

I quit using trailboss for this purpose. Worked, somewhat, just a wide spread on velocity. Really wide with a 223. Drilled out the flash holes and use solo1000 since I have a pile of it. No first shot weirdness, very low spread on velocity. Use a 200gr lyman powder coated, or 80gr in a 223.
Rocketman59
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Re: Subsonic 308WIN Trailboss 180gr loads - 1st shot in strings low velocity

Post by Rocketman59 »

I am on my second try for Subsonic 308's. First try was 11.0 Gr of Trailboss in Surplus Military (LC) pushing 180 Grain Sierra RN, Win Mag Primers. Did not notice first round low velocity. Avg for this load 1150FPS. Definitely were supersonic as I could hear snap going down range. Next try was 10.5 Gr of Trailboss, same Brass and Primers. Now getting 1050 Avg FPS in Ruger RPR. No snap going downrange. Going to load some more up with this combination.

Also found this Speed of sound chart on line. My altitude is 600', Temp today was 55deg.

https://www.engineeringtoolbox.com/elev ... _1534.html

Working on 223 load for Subsonic. Using 75 Grain, Midway Blems JHPBT. Hornady Civ Brass using CCI 450 Mag Primers. OL 2.240. First try 5.5 grains Trailboss. Avg was 999.6 FPS out of Mosberg MVP 19" Barrel, 1:7 twist. Was very quiet out of LaRue TranQuilo suppressor. Think I will try going up to 5.7 grains and see what I get.

Also had issues getting consistent Trailboss powder weights out of my Dillon Powder Measure. So used a Lyman Generation 6 powder measure system and weigh each charge.

Good Luck,
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