Max bullet diameter for British .303?

[spinecracker]

I recently slugged the bore on my 2-grooved 1941 Long Branch No.4 Mk.1, and it came out to a groove diameter of 0.315" and a bore (lands) diameter of 0.303". I have been using Sierra MatchKing .311 174gr HPBT with 38.3gr of Varget, Prvi brass and CCI large rifle primers. 5 round group size with this load at 100 yards is a consistent 1". I am considering seeing if a larger buttlet might tighten up group size a little, so I will be trying out the Hornady 0.312" 174gr BTHP bullets, but I was wondering what would be the maximum safe bullet diameter for my situation (e.g. the minimum difference between groove diameter and bullet diameter)?

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[vintage hunter]

You getting 1'' 5 shot groups @ 100, don't know as it can get any better than that. I tried the Hornady 312 dia bullets, includind the 150gr Spr Pt and 174gr FB RN hunting bullets, and got poor accuracy and excessive copper fouling in my LB. A 312 bullet has to swage down a good bit (.009'') to pass through the .303 bore and the narrow grooves don't give much room for the metal to flow, I've noticed that recovered FB bullets show an uneven skirt at the base as the metal had to have someplace to go after the grooves were filled. If you decide to try them I suggest reducing the charge by 1 1/2gr and work up as the larger bullets will cause an increase in pressures. My '43 LB 2 groove gives the best accuracy is with Remington 180gr .310 dia. RN's and IMR 4064 powder.

[spinecracker]

Thanks for the reply. This is why I ask before I do something bloody stupid. I was thinking of reducing the amount of powder anyway, but I hadn't made up my mind. I have now.

[JBS]

Have run into the large bore before. One customer had a bore of .318 over .303 He ended up using a cast bullet that had stepped driving bands. Its called a Christmas tree design in the cast bullet world. The fist driving band was .315 and the last was .319 it produced excellent groups. Just another option.

[Patrick Chadwick]

5 round group size with this load at 100 yards is a consistent 1".

Don't worry, be happy. Very happy. That must be one of the best shooting Mk1s around!

"The search for perfection can damage that which is already excellent."

Patrick

[spinecracker]

Well, I tried it with the Hornady .312 bullets. At 100 yards, it was about the same group size as the Sierra .311 bullets, but at 200 yards it opened up a bit more (~3" versus ~2"). I think I'm going to stick with the Sierra Matchkings. My 91/30 will get fed the Hornady bullets lol.

Patrick, I was shooting from a sand bag rest, and the rifle has been scoped (Fulton's "no-gunsmithing" mount). The day was perfect, with no wind and no haze. Heck, I've shot that rifle at 100 yards with iron sights in a blizzard with surplus .303 ammo, and STILL got a 1 1/2" grouping.