Gunsmithing the model 788 Remington

Shooting Industry, May, 1990 by William Schumaker

Gunsmithing The Model 788 Remington

Discontinued Remington M-788 rifles which originally cost about $90 handle such cartridges as the 6mm Remington, .22-250, 7mm-08, .257 Roberts, .308 Winchester and the 7x57 Mauser (rifles were also chambered for the .223 head-size calibers). Their bulk, weight, rigidity and minimal magazine and ejection port cut-outs gives them many of the attributes sought by the super-accurate benchrest shooters.

Why not try a M-788 chambering as a days-off gunsmithing project? The 6mm BR Remington chambering reamers are available from Clymer Mfg. Co., Inc., 1645 West Hamlin Road, Rochester Hills, MI 48309-3368.

Accuracy was a must. From my target rifles through varminters, I wanted to high-grade a little of that extra super benchrest and apply it to a varmint-weight barrel in a rigid sporting rifle. Due to the high cost of match-winning .22 and 6mm PPC ammunition, I wanted to give the American-made 6mm BR Remington a try. I purchased a 500 round case of Remington loaded ammunition, sufficient for myself and several customers.

Due to the existence of a wildcat 6mm BR and the 6mm BR Remington, orders for the reamers, gauges and loading dies have to be carefully stated. Remington 6mm BR ammunition boxes are marked: NOTICE: This 6mm BR Remington cartridge will not fit in 6mm BR chambers. Remington thickened the case neck walls and lengthened the neck .040. This makes it impossible to use the Remington commercial 6mm BR Remington ammo in any of the wildcat 6mm BR chambers made earlier or now.

The wildcat 6mm BR cases are 1.520 inches maximum length and 1.510 as normally trimmed. The neck wall thickness is .011 inch. Neck wall thickness on my lot of 6mm BR Rem. ammo measures .013 inch average. Therefore it is extremely important that none of the commercial ammo be fired in the shorter, tighter wildcat benchrest custom chambers.

Fitting A New 6mm BR Rem. Barrel

Fitting a Douglas 1-in-10 twist barrel to a used Remington 788 consisted of removing the original 6mm barrel and the usual turning of a shank diameter of 1" x 1.561" shank length, threading 20 tpi and careful chambering with our new Clymer reamer, which was plainly marked 6mm BR Rem., as were the gauges.

The bolt closed hard on the first factory round, and lifted with difficulty after firing. Several more shots indicated excessive pressure and bolt sticking. There was even a slight swelling of the case head brass into the ejection plunger opening of the bolt face! We stopped shooting, pulled several of the factory 100-grain bullets, reduced the factory powder charge 1-1/2 grains and substituted 75-grain bullets.

Pressures became more normal, but accuracy was poor. It was impossible to slip a .243 bullet into the necks of the fired cases, indicating that the cartridge neck was too thick to provide expansion for proper bullet escape.

Insufficient Chamber Neck Clearance

A gun enthusiast friend who lives, reads, eats and sleeps with bench rest shooting tolerances, and information from books and articles, suspected the reamer might have been marked wrong, and that we had the wildcat reamer! Our measurements didn't bear this out, but after making a chamber cast and measuring the neck diameter it was found to be .271", very close to the accepted .272, mentioned to me by barrel-maker-bench rest shooter Ed Shilen.

We were fast discovering that gunsmithing with bench rest calibers required everything to be held to closer tolerances. The chambering reamer had cut a very nice minimal chamber, allowing very little case expansion upon firing. The Forster full-length bench rest sizing die did a nice job of sizing fired cases for chamber re-entry. A Forster outside neck turning tool was used to bring neck thickness to .010". Shortening the necks so case length was 1.515 eliminated all possible case length problems.

Although the case neck doctoring was producing good accuracy results, equal to or slightly better than my best varmint rifles, we were unhappy with the commercial 6mm BR Remington caliber reamer not cutting a chamber to safely fire the Remington ammunition. Obviously their 100-grain soft point bullet loading was intended for hunting purposes, (not bench rest). After reading my tale of work and woe, the Clymer reamer folks asked that I return the reamer and headspace gauges for a close inspection along with a sample of my lot of factory ammo.

Phone-back information revealed the reamer was made in accordance with ammunition specs they had at the time, but my lot of ammunition was thicker necked and slightly longer. They suggested making a custom reamer for the ammunition, as surely I was interested in chambering 6mm BR Rem. barrels for customers. I suspected such a set-up might merely be a another varmint cartridge, minus the close bench rest tolerances.

The project is on temporary HOLD! The gauges and another reamer have been returned, and in all probability I'll keep altering cases to close tolerances and having fun! The present 10 twist barrel is admittedly wrong for the short match bullets of 69 to 70-grain weights. I plan to re-barrel this or another action with a Shilen No. 5 heavier 12-twist stainless steel barrel, as I like to use 75 to 85-grain hollow point bullets. However, the accuracy fellows advise that a 14-twist is most ideal for the 69 to 70-grain match bullets.