With my next planned step being to run the series of Tubb Final Finish bullets through my rifle, I wanted to confirm my bore and grove diameter so I could order the proper diameter Tubb lapping bullets. In load development work for this rifle that I had done, I could never get .308 diameter bullet to shoot well at all, and I found that .311 diameter bullets produced the best (although not good) results. I had previously slugged the barrel and determined it was closer to .310 grove diameter, but given all the copper removal I had done and the removal of a couple of sizable 'sheets' of lead from the bore, I decided to repeat the slugging process. Without a special V-micrometer it is not possible accurately measure the groove diameter of a 5-grove barrel, but by carefully rotating the slug inside the jaws of a caliper, the caliper will open up to approximately the circular diameter of the grooves. I made 2 slugs of the rifling, and using the noted caliper process I came up with a diameter of .310 or a little over. As a second check, I drilled a hole in a piece of scrap steel with an N drill bit (0.302" diameter), then using emery cloth on a mandrill I carefully and slowly enlarged the hole in the steel until I could just get a .308 diameter jacketed bullet to pass through. I determined that neither of the slugs I had made would pass through this hole, so this confirms my grove diameter is larger than .308. I continued the process of slowly enlarging the hole in the scrap steel piece until the slugs would just pass through. Measurement of this hole with my calipers showed it to be just a tad larger than .310. So this indicates I need to use the .311 diameter Tubb lapping bullets in order for them fully 'scrub' the bottom of the grooves.
A second test I made was to try to determine the bore diameter of my rifle. The forums are full of opinions regarding the bore diameter of the 1917 Enfield, some saying they were spec'd and produced as a .300 bore rifles, and others contending that some rifles were made using surplus barrel stock that was left over from when US manufactures made this rifle for the British in the 303 British cartridge, resulting in rifles with a .303 bore. Some contend that in the rush to produce rifles for the US Army when the US entered the war, manufacturing tolerances may not have been strictly adhered to. So to answer that question for my specific rifle, I machined a brass sabot I could attach to my cleaning rod to pass down the bore. I started with a sabot machined to .3030" and it would not pass down the bore. I kept gradually reducing the diameter of the sabot until I could get it to pass through the bore, and measured the diameter with my micrometer as .3017". This suggests my rifle started life as a .300 bore rifle (more or less) and the lands diameter has increased by 0.0017" during its one hundred year-plus life. Given my determination that I have a .310 groove diameter, this means I have rifling that is about 0.004" deep which should be more than adequate for gripping bullets.
I then decided to lap the bore to see if I could clean up some of the 'alligator-skin' looking fouling that remained. I did not have any JB Bore Paste that was suggested to use, but I have some 800 grit lapping compound. I first started by using the brass sabot coated with the lapping compound and made several passes through the barrel to until the sabot would pass easily. My thought was that this would 'scrub' off the tops of the lands. I then switched to a cleaning patch coated with the lapping compound, wrapped around a bronze bore brush and made about 25 passes through the barrel. After thoroughly cleaning the barrel, I examined with my borescope. This definitely cleaned up many areas of the barrel, but there is still evidence of the 'alligator-skin' fouling in many areas. I am undecided as to whether I want to do more lapping at this point or just wait until I fire the series of Tubb lapping bullets.
There is a lot of pitting which will remain even if I can remove all the 'alligator-skin' looking fouling.