308 subsonic advice?

[Sniper338]

Well i had a subsonic load shooting pretty well... under an inch at 80 yards..175smk and 10.9gr of trailboss..

Today i shot, little cooler outside.. but a 5-6 inch "spread"... it was all vertical stringing...

Ive never had an issue with vertical stringing... they were all the same load...

Only thing i can think of is powder laying in the case differently creating different burn rates/pressures... but that font neccesarilly make since to me either as these loads shot well for me before.. only a 30 degree temp difference... 90s vs the 60s..

 

[Lapua40X]

...
Today i shot, little cooler outside..

How cool is a "little cooler"?

....
but a 5-6 inch "spread"... it was all vertical stringing...

What's your setup (rest, sled, rear bag, off hand?)

... Ive never had an issue with vertical stringing... they were all the same load...

Look first at the shooter, then the setup, then the load/rifle

 

[savagedasher]

What was the bullet speed from one day to the other ? Larry

 

[Intheshop]

Shooter,then rest/setup,rifle,...gets attention in that order.Then we tailor the load.

I shoot mainly cast these days.The above becomes critical because of the increased brrl time.

Bump powder up to get round groups/node.But if you have anything wrong in your bench manor's,it's gonna show up in spades,IME.

 

[Sniper338]

Shooting off my rempel bipod and gator rear bag... proned out.. pretty solid...

30+- degree temp difference... trailboss powder... not sure how stable trailboss is..

 

[Sniper338]

What was the bullet speed from one day to the other ? Larry

This here, i havent chronoed any of these subsonic...

 

[savagedasher]

This here, i havent chronoed any of these subsonic...

Temeture can effect that powder vertical is bullet speed normally . Larry

 

[Canadian bushman]

I wouldnt be suprised if temperature was the culprit.

If you have any left shoot them on a warm day and you will know.


If you wanna be picky, load the gun pointing at the sky and slowly lower onto target. This usually adds a little consistency to ES with rounds with a low fill %.

 

[mgunderson]

I have been shooting a similar load with the SMK 175 and trailboss. It has been stable when shooting at 80F to 20F. You may want to consider what went through the barrel in the previous shots. Your sub 1 inch group is better than I generally get.

 

[SheepDog]

If the loads have been sitting for a while there are a number of things to consider. If they are fresh loads then there are fewer things to consider.
Lead hardens/softens with age depending on the alloy. Lead and brass will bond over a very short time causing more pressure to release the bullet. Cast bullets are notorious for hidden occlusions and balance problems. Your powder choice is great, Trail Boss is one of the best for subsonic loads. It is not particularly sensitive to volume fill and is bulky enough to be insensitive to temperature. Clean dry barrel is a must but even more so for lead bullets. The lower velocities makes the load more sensitive to "crowding" or "cheeking" the stock. the cold weather forces you to bundle up more and that reduces your feel for the rifle. Repeatability of your position and approach to your rifle can change the point of impact a lot. If you are using very long bullets the temperature drop will adversely affect stability and thus grouping.

These are a few of the things to consider. It is best to work on you first and then one thing at a time to eliminate all of them.

 

[Lapua40X]

Shooting off my rempel bipod and gator rear bag... (*) proned out.. pretty solid...

30+- degree temp difference... trailboss powder... not sure how stable trailboss is..

(*) Right there could be your vertical.
If "pretty solid" implies pressure on the comb of the stock you're interfering with recoil; a vertical stringing causation factor. I read some place that a shooter was using a piece of closed-cell foam held on the stock with duct tape and that it was a good "head rest". Riser Pad, Cheek Rest, whatever you choose to call it, should not be usesd as a "head rest".

 

[Berger.Fan222]

Consistent velocities are tricky with TrailBoss. One day it seems to work, the next day it doesn't. Bore condition can also impact things with light loads. Subsonic consistency is tricky business.