I spoke with UM today about this project, but I'm simply looking to replace a factory Tikka T3x .270 WSM barrel with an identically contoured drop-in. I'm partial to that dark laminate stock, and accuracy ought to be good enough, I reckon out of that sporter barrel.
I'm curious to know if you've gotten your barrel from UM yet. I see this post was about 3 months ago.
I developed my own 6 SAUM 15 years ago from the 7 SAUM parent case. We didn't have nearly the options of rifle or reloading components available today. As I recall, I used a Redding .25-.300 SAUM die for the first pass on Remington brass, and them a Redding 6mm/300 SAUM die to finish. Had lots of issues with bending case necks on the old Remington brass due to the metallurgy and the fact that Ibwas running a .300 SAUM-based die with 7mm SAUM cases, which have longer necks. I had to turn necks, also. I had it chambered in a Remington Sporter weight Lilja 3-groove barrel, and eventually got to 1/4 MOA, but was never able to come near it again. Accuracy deteriorated seemingly in 80 - 100 shots. I decided maybe my luck with the 1/4 MOA was a lucky fluke. I didn't have access to a bore scope. I concluded the case was too overbore for the 6mm realm and remade another in .257, which did much better. I used a sleeved Krieger #2 barrel for this one, though. That may have had lots to do with it.
I had 105gr VLDs and 115gr VLDs, but experimented almost exclusively with the 105s. Speeds up to 3400 in my thin 24" barrel were possible. For hunting, I decided on Sierra ProHunter 100gr flat bases.
That bullet in the 6mm SAUM (I dubbed the .245 Sabertooth), was an artillery piece on game. I could get 3450 or a little better out of the 100gr (those have a thicker jacket than normal, btw), but settled on 3350 ft/s for best accuracy. I completely blew up a 65lb coyote at maybe 100 yards. It literally put a hole larger than a softball through his neck. I was able to shoot 1 smaller 7-pt buck at a mere 20 yards with that load. It picked the deer up, flipped him 180 degrees head over feet backward (he was quartering to me), had a small hole going in, a slightly larger hole going out (high shoulder), and very little meat damage. Narrowly missed the heart and barely clipped the top of the lungs, but his spine was ruptured about a foot above the wound channel. Hydraulic shock was incredible! Spinal fluid leaked out when I hoisted him up. Things would have been SOOO much easier with ADG (or similar) brass and 6.5 SAUM cases and bushing dies. I'm still of the opinion that a 6mm probably ought to stay below 70 grains of water for maximum case capacity. I passed on their (UM) 6mm UM. I've seen that around the 3300 ft/s mark with 100-ish grain bullets, BAD things happen to live targets. I could probably get close to that with a long-throated 6mm Remington with enough barrel.
I would guess the 6mm PRC would be closer to the 6mm-06 AI than the standard 6mm-06 because it about splits the difference in case capacity and it operates at 5,000psi higher pressure than the standard '06, I believe. Certainly, there are the advantages of the shorter case, including more consistent powder burn.
I would prefer a 5R or 6-groove for something like this. It seems often the more grooves, the less bearing surface/friction and the more speed. This may not always be true, and I'm aware the more grooves, the more fouling. Quality lapping can mitigate much of that.
I believe, for my money, the 6 PRC (unaltered from parent other than neck diameter) is about at the limit of the amount of cartridge I want to fire from a .243" platform. It would make a great replacement for the factory .270 WSM barrel in the lightweight Tikka Stainless Laminate, as well. I recall there was a modest amount of recoil, even in a sub-7lb platform I used 15 years ago.
I'm interested to hear how your experience with Unknown Munitions on this project is going! Great thread!
