Possibly a little more info than you wanted, but here you go ;>)
M40A1 Barrel profile:
There have been several barrels throughout the course of the M40a1 in varying lengths. The dimensions are as follows: 1.2" for 4" then tapering to .91-.92" at the muzzle. These barrels were 6 groove, 1/12" twist, stainless steel and finished with a counter-bore crown. Hart, Schneider, Atkinson/H&S Precision all supplied barrels at one time.The lengths vary from 26" to 24" measured from the front face of the recoil lug.
According to Gary Schneider of Schneider barrels the M40A1 barrel dimensions were 1.10-1.210 +/- X4", straight tapering to .890-.910 +/- @ 26.5".
After cutting and threading the breach end you would end up with 1.20" X3", +/-.
Once it was threaded at the breech end they the USMC armorers would trim the muzzle of the barrel to 24" from the front of the trued recoil lug, as is their method of measurement, making the muzzle approximately .920-.940, again +/- . ;\)
He stated there were variations of +/-.015", maybe more at times.
He also said the barrels had a rough turned finish when supplied to the Marines Corps initially as they were rushed to meet the deadline. He said he offered to make them smoother on production models if accepted(which he was), but the Marines liked the rough finish as they said it held paint real well, so he made all the rest of the contract the same way.
As for the curious 1.20X3" finished breech end measurement ,he says that was strictly to facilitate the use of the night vision equipment they had/have. Any longer or thicker in the breech end would cause interference with the objective, or other parts of the NV equipment. So it wasn't just some outdated military quirk. Most factory or replacement rifle barrels usually have measurements of 1.25"X 4-5" X whatever length.
