I was in the same situation as the OP, and also did all the playing around on QuickLOAD to see about whether it would be worthwhile with 150s/155s, generally deciding that there really isn't any situation that would suit this set up, or at any rate produce worthwhile benefits from it. Barrel life aside, the 6BR will always outperform 7BR which will always outperform 30BR in the 300 yards and beyond roles. (6.5BR or 6.5PPC = Grendel might be an interesting choice here though!)
As per Sam, the OP, I had all the bits for a fast twist barrel single-shot rifle - an old 4-lug Paramount TR (Fullbore to you guys) rifle, the immediate predecessor of the RPA Quadlite in a superb walnut prone stock with a very high round count, much eroded throat 1:14 twist Lothar-Walther Palma profile job that still shot surprisingly well at short ranges but had lost a great deal of MV and gained a lot of ES points. I'd got it for a poor man's F/TR rifle project that had fallen by the wayside and after a lot of chasing around, the UK's Tier-One outfit had made a custom 20-MOA scope rail for it.
The obvious thing to do would have been to rebarrel it in 308 again for F/TR and sell it, but the likely selling price would have barely covered a gunsmith rebarrelling job and the mandatory EU proofing and I'd have been well out of pocket for the original rifle cost and the rail. Anyway, I liked the rifle, so wanted something I could use as a cheap, low barrel wear 'old geezer's range plinker'.
As luck would have it, Bruce Lenton, one of our top BR competitors fancied a 30BR and built a 'proper' (short, slow twist barrel, suitable action etc) HV weight rifle buying a Robinett chamber reamer, that's a no freebore job for 115-118gn FB short radius ogive jobs and I gave him all my spare I&H 4198, it being as common as hen's teeth here. (Now banned in Europe too under EU Project 'Reach' on health & safety grounds.) We don't have short-range BR for score in the UK, so Bruce was experimenting with it as an alternative to 6PPC. AFAIK, the results were superb, but the heavier recoil made it unattractive compared to the six especially with our main 100 yards BR range, Diggle 'A' having some of the twitchiest, hardest winds on any range BR is shot over, conditions that can sometimes induce near nervous breakdowns, so fast shooting is usually vital!
I also had a virtually nil round-count 10-twist Krieger available from an AR-10 straight-pull project that had been a disaster, a suitable scope or two with mounts, lots of odd part lots of powders that QuickLOAD says will (sort of) work with the 30BR - Viht N120, N130, N133, Re10x etc - 6BR brass, plenty of SRM primers and a great gunsmithing and shooting friend Vince Bottomley, so the only new outlay was bullets and dies. Bullets were a bit of an issue - ideally I wanted Berger 115gn class pills but they were virtually unavailable here. Sierra 125gn HPMs and Berger 150 FBs were readily available though.
The rifle was put together and run in late last year, but thanks to health issues I've had virtually no opportunity to do any proper load development this year. The first thing that appeared though - quite unanticipated so I give it as a warning - was very poor feed in the Paramount action because of the BR being so much shorter than the 308 and so nose-heavy. Pop a 30BR into the port and it lies on the action floor with its bullet tip down touching the floor. If you try to close the bolt, the tip butts up against the barrel face well below the chamber. So, it becomes a very fiddly feed/fire job. This cartridge needs a proper short BR action of a feed ramp.
With the no freebore chamber, as previous posters have said, you really do need flat-base bullets and you run with powder well up the neck and compressed charges, especially with many of the powders I intend to use up being slower burning than the 4198s. You need a lot of neck tension too, and I've not had nearly enough so far - when I get going again that'll be the number one change / action - another couple of smaller die bushings and a lot more bullet grip. The backstop was always a run-in with a throater to use 155s, but I really doubt the heavier bullets' benefits even in the short-distance plinker role and the external ballistics' benefits are small to non-existent over 125s.
The first (and only to date) proper range session with 125gn SMKs jumped a little and N120 saw groups of 0.5-0.7 inches and the top charge of 30.5gn N120 give a modest 2,868 fps MV from a 24.75" barrel. Spreads ranged from single figures to 20 fps. The next go will take charges up to 31gn, use a great deal more neck tension and seat the bullets out a little to be 'in the lands'.
As I said we can no longer get the 4198s here at all (thank you Brussels!), but I've sourced some Lovex SO53, a Czech manufactured extruded short-cut single-based propellant designed for the 7.62X39mm with an apparently similar quickness, and will give that a try in due course, also some 115gn Bergers. (Hmmmm ... what happened to the original budget rifle idea of just use what's in the cupboard?)
This will never be a benchrest outfit, but should be a cheap and fun, low recoil 300 yard bench and prone rifle. If it shoots well enough, I'll give it a try in 300 yard F matches on 'quieter' days when out and out accuracy trumps high BC wind-bucking bullets as an alternative to 6BR.
If the Sierra 125s don't work, I bought 500 Speer 125 TNTs. My fallback after them are my custom 118s. I have plenty of 308s to use the 125s in for training at short range and for my friends, the ammo hounds.
