Transonic 308

[oaken]

I am looking for information on various .30cal 155gr bullets and their ability to remain on track through the transonic. Personal experience is that scenar are poor and Sierra (both) are bad, I'm too tight to pay for the 155.5 bergers.
I have a TRG with 26" barrel that just can't generate enough safe bullet speed to maintain stability (and hence accuracy) beyond around 900yds.
Thinking laterally, a bullet that is able to transition to subsonic with a degree of stability could be a way forward.
Thanks
Rup

[The Gun Pimp]

I am looking for information on various .30cal 155gr bullets and their ability to remain on track through the transonic. Personal experience is that scenar are poor and Sierra (both) are bad, I'm too tight to pay for the 155.5 bergers.
I have a TRG with 26" barrel that just can't generate enough safe bullet speed to maintain stability (and hence accuracy) beyond around 900yds.
Thinking laterally, a bullet that is able to transition to subsonic with a degree of stability could be a way forward.
Thanks
Rup

It's not necessarily a problem - I remember once a shooter with a 308 shooting 1000 yard benchrest. The message came back from butts "His rounds are going through sideways - but it's a five-inch group!"

I don't remember the bullets but they certainly weren't Berger - they didn't make a 308 bullet back then. I would guess at Lapua Scenar.

[TJC]

Perhaps some 178gr AMAX or even 190 SMK but it all depends on how you load them. What MV are you getting with the Scenars ? You ought to be able to get them to c2900 fps from a 26" TRG. Are you seeing pressure signs long before there ?

[ovenpaa]

The Lapua 170 Lockbase works well to 1000 yards from a .308 - a stout load of N550 is the recommended load however I had better results with N540. This was my 'go to' bullet in my AI for quite a while.

[IainWR]

The critical issue here isn't always stability. Velocity consistencey has a lot to do with it.

In the transonic region (M 1.2 - M 0.95 or thereabouts) the drag is vastly higher than any other velocity range. That means that the deceleration is vastly greater for that bit of its flight than for any other bit. That in turn means that the time available for vertical motion under gravity is greatest for the bullets that arrive slowest. Now if the relevant Mach range happens entirely outside the critical distances of 900, 1000 and 1100 yards (assuming we're at Bisley on Sickledown and not on a metric range), then every bullet suffers the same deceleration in the transonic zone and the effect on the group can be minimal. However, if the Mach range brackets one of the critical distances, then the vertical scatter due to small velocity differences will be much larger than for bits of the flight where the drag is lower. It should be obvious that this issue is entirely independent of the bullet stability in the transonic range.

In the long term and speaking as a TR shooter who routinely has to contend with this, the answer is a longer barrel. Every extra inch will give you another 20 ft/sec or so muzzle velocity (yes someone did the experiment by sawing an inch at a time off a barrel starting from 34", and the results are in the NRA Journal sometime in the 1990s). A 30" barrel will give enough MV to keep a Sierra 2155 above Mcrit for 1000 yds under all but the most adverse conditions, and the 2156 and Scenar will both cope better than the 2155.

[TJC]

Every extra inch will give you another 20 ft/sec or so muzzle velocity (yes someone did the experiment by sawing an inch at a time off a barrel starting from 34", and the results are in the NRA Journal sometime in the 1990s).

Thankfully, the world has moved on and with the development of superior powders, primers, bullets and especially barrels you can comfortably get out to 1k with a 20" .308 these days.

Hell, you can even do it with an 18" and match ammo. Check out Frank Gali doing this with his tight bore GAP Gladius.

http://youtu.be/15qj032UJ1I

[oaken]

Vince. They are not going sideways (to my knowledge) but there is a considerable difference in grouping 900-1000 especially with a hint of wind.

TJC. I can't get anywhere near 2900. Anything over 2750 produces pronounced ejector marks on the brass. That's with anything over 43gr Varget, Lap brass and 91/2 primers.

Davy. As above, a stiff load is not really on the cards.
Not heard of the lock base, where would I find them?

Iain. Good point about velocity variations, but last time I chronoed a string of 10, I only had a 20fps spread with the load mentioned above. I anneal my brass which seems to give reasonably consistent results.
The load mentioned above contrasts dramatically to my old Ftr (30"barrel, now sold) which would handle 46gr Varget with ease.

I suspect root of my problem is hoping for TR accuracy beyond the realistic range of the rifle.

Going to try a slower powder next.

[TJC]

I suspect root of my problem is hoping for TR accuracy beyond the realistic range of the rifle.

I'd suggest the root cause is a rifle that can't even manage to reach the 44gr start load for a 155gr that Hodgdon recommends and we know start and max loads have quite a bit of protection/safety built in. With material pressure signs at sub 44gr Varget I would get someone qualified to take a good look at it.

[tackb]

I used 50g of H380 under a 175mk for 2750fps in my TRG21 , hot but ok in my rifle so I'd expect more from a 155 ?

This was a lot of years ago though!

[oaken]

Hmmm that may be a fair comment. I do know other TRG owners that can and do run a lot hotter than I seem to manage.
Apart from ejector marks, the brass is not being misshapen at all with no headspace issues. Seating deapth seems to be consistent with other TRGs. Bolt seats snug with even wear on all the lugs. Not sure what else a gunsmith would look for?
It is possible that there is an issue with the powder, I got an 8lb tub shortly after getting the rifle and don't recall pressure problems with initial load development. Though no problems in other rifles that I use the same Varget in.
Maybe I should get another tub of Varget and do some more load tests looking for pressure.