Long Branch 69L Mk 1/3 gets a new Barrel

Ideallly I'd like to get a Criterion .30 1/10 blank, have it turned to the L39/L42 barrel profile.

I opened up the box that my Lothar Walther barrel was shipped in, weighed and borescoped the barrel. Here is what I found:

Barrel weighs 2 lb 8.75 oz. That’s about right on spec.

Rifling is 4 groove, RH Twist. Grooves appear to be a bit wider than the lands and are square edged.

The bore and the throat has obviously been hand lapped to a very good finish. I would not put a reamer in this chamber either. 

Photo below is at the front of the chamber, bottom portion is the end of case neck annd top portion the origin of the rifling. 

This photo of the throat, just slightly forward of the previous photo:

Some of those barrels are on exceptionally tight.  I had one where we actually twisted the barrel  trying to unwind it.  If the barrel is junk undercutting it in the lathe is a good option for removal.  Breeching-up washers can of course be made to reuse even undercut barrels.  

Perhaps the best method of aligning the back and front sights when breeching-up is to put in the backsight cross pin and bridge the gap from the pin to the charger bridge with a gauge block or a 1/2" square HSS cutter or some other precise and square piece of material, then sit a small precision level on that.  

For the barrel, putting on the foresight block, pinning it and then using a precision magnet across the top of the sight base with another level would be ideal, since that would compensate for any inaccuracies in the alignment of the foresight block.  Working from the flats of the Nock's form is almost as good though.

Are the Walther barrels cold hammer-forged Britrifles?

No, button rifled.  I still have one in the box. Borescoped it and they did a very good job reaming the throat, looks like they lapped it. They are supposed to be 0.01” short chambered like the CBI barrel, but a 174 gr SMK has quite a bit to jump to reach the lands (more than 0.10” IIRC) when seated to magazine length. 

Both CBI and Lothar Walther are using SAAMI specs for the chamber, although they are supposed to be reamed 0.010” short to allow for headspacing on installation (which is rather odd since headspace is set by the rim, and can be adjusted by selecting the bolt head).  

On the last two CBI barrels I installed, I did not ream the chambers at all, so they are “tight” compared to military barrels, particularly in the length of the chamber from breech to shoulder. I took an old case trimmed to 2.222” long, bored a hole thru the primer pocket that will pass a borescope. I found the case mouth was within a few thousands of an inch from the end of the chamber when the case rim contacted the back of the barrel. Fired cases don’t get the shoulder blown nearly as far forward than they do in military chambers.  

I’m not sure how many rounds these .303 button rifles barrels will last.  Based on my experience with .223 button rifled barrels, I would not expect more than 3,000 - 4,000 round accuracy life.  My .223 button rifled barrels only gave good accuracy for 1200-1300 rounds, but these are hotter loads with quite a bit of rapid fire use.  I was also using a double base powder which typically burns hotter than single base.  

I don’t think I’ve come across a military .303 LE chamber drawing, but with these rifles being manufactured at numerous locations for many years, I’d expect a lot of variability.