Late to the party, but Ill give my $0.02 anyway.
You aren't having great luck with 55 gr and 62 gr bullets because bulk 55gr and 62 gr FMJ bullets generally stink. They aren't going to be great performers in the best rifle made.
Try some better match or nicer varmint bullets. Sierra 55...
Couple of things:
1. Awesome to see someone else getting into highpower! When you're ready to come to Atterbury next year and shoot with CIHPRS, let me know.
2. Any if the powders you mentioned will work ok, but don't overlook Vihtavouri N135 and N140. Its top notch stuff, and somehow more...
I dont have any experience with the CLE float tube, but enough good service rifle shooters use them that if there were an issue, it would be well known by now.
I have 2 of the Rock River tubes. I run my sling tight enough that I'm bruised up pretty good after a 60 round midrange match, even...
Zermatt Arms actions (Origin, TL3, SR3) are all CRF. Great actions....(relatively) affordable, 700 footprint and triggers, Savage barrel compatability. Shouldered prefits available too.
As mentioned above, CIHPRS hosts 4 matches a month at Atterbury...1st weekend of the month is midrange Saturday and a 3x1000 Sunday, 3rd weekend is a 3x1000 Saturday and a Palma match (800, 900, 1000) Sunday.
https://cihprs.wildapricot.org/
PM me if you'd like more details. I'm on the board.
I can check to make sure, but there might be a minimum age. There are ways around it. Anyone under 18 will have to sign a waiver I believe. Not necessarily our (CIHPRS) rules, but the base's rules.
Baron James shoots F-Class with us at Atterbury/CIHPRS and builds precision rifles. I don't know him well, but his rifles obviously shoot based on his scores.
https://totalprecisionrifleworks.com/
What dimension are you seeing the variance on? Head to shoulder, or head to case mouth? I'm assuming this is a rifle cartridge...regardless, a few thousandths isn't worth worrying about as long as it chambers, and ESPECIALLY if its a pistol case, or if you're measuring overall length. If its...
A 1:8 twist will stabilize anything up to and including 80 gr Sierras, which have to be single fed (too long to fit in a magazine). 69 gr matchkings, 69 gr rocky mountain reloading bullets, 73 gr Hornady ELDs, 75 gr Hornady BTHP, 77 Sierra and Noslers would all be great bullets in your setup...
I'm on the BOD for CIHPRS. We only shoot at Atterbury. We generally shoot every other weekend (mostly) from March to October. Usually 2nd weekend of the month is a midrange match on Saturday (20 shots for record at 300, 500, 600 yards) and a 3x1000 (20 shots per string, 3 strings for the day)...
I shot my Ruger Precision using 88 ELD-M's in the Ireland USMC match in 2019. One of those $!#@ Illinois guys that like to come **** on our parade had a match rifle in .224 Valkyrie, 26" barrel, and he was shooting the Hornady factory 88 ELD ammo. His rifle was throated to put mag length 88's...