Today received back from John Taylor one of my small H&R .44/.410 receivers fitted with 22" barrel chambered for our 10x25mm Rimmed wildcat. Our fantasy concept was a modern, RIMMED .40 cal. cartridge, using either .40 S&W or 10mm jacketed or .38-40 cast bullets. It would serve the medium game “packing pistol” role, approximating .38-40 black powder rifle ballistics with smokeless, from a sturdy revolver, in a smaller, sturdy case, formed from common brass, avoiding the excess free airspace and fragile nature of .38-40 brass. A case full of black powder or Trail Boss would be adequate for Cowboy Action, plinking, or small game at about 700 fps with either round ball or bullet.
Once the concept was tested and proven in the revolver, I wanted a companion rook rifle or take-down backpack gun chambered for the same round. The .44 Russian case necked down to .40 has a capacity of 20-21 grains of 3Fg, 4.0-4.5 grains of Trail Boss using the RCBS Little Dandy Rotor #18. From our custom 5-inch Rugers, smokeless performance with 5 grains of Bullseye powder exceeds .40 S&W factory ballistics.
Dave Manson made the reamer. John Taylor first rebarrelled a couple Ruger Blackhawks using a Green Mountain “gunsmith special” 10mm barrel with 16” twist, rechambering one cylinder for each gun to .40 S&W, and the other to 10x25mm. .
We start with Starline .44 Russian brass, which we neck down and use as-is, resulting in a 0.970-0.975” case. The shoulder angle is 6 degrees, 48 minutes Basic, the same as the .38-40 Winchester. Case body diameter is .454" at the shoulder, the same as the .38-40 and .457" at the base, and the same as the .44 Special.
“DougGuy” made our loading dies by cutting down and honing out .38-40 Lee dies on his Sunnen hone... Doug’s firm price to modify your set of LEE .38-40 dies to 10x25mmR is $150. You supply Lee .38-40 dies, Doug does the mod, and returns your dies insured in small flat rate box. A shortened Lee Quik-Trim case holder, to cut .44 Special brass, costs a bit extra, but isn't needed. The .44 Russian brass is a little wee bit short of 1", but is much easier and works great! Once you have fire-formed brass, you can neck-size only by using a 0.525” spacer with your .40 S&W Auto die set.
Anyone ordering this reamer from Manson should specify .40/.44 Special Short Harris Rev1-6/18. Chamber has .429” neck diameter, with a .402” cylindrical ball seat 0.10” long with 6 degrees, Basic forcing cone. This permits using the Accurate 40-220H bullet as-cast and unsized in either a rifle or revolver chamber. Revolver chambers are roughed with .4015" chucking reamer frrom McMaster-Carr then finish-reamed with the Manson reamer.
I fired Starline .44 Russian brass with 180 Hornady FMJ, Accurate 40-182H and 40-220H, again, all with 5 grains of Bullseye. From the 5-inch Ruger 180-grain Hornady FMJ with 5 grains of Bullseye give 932 fps with a standard deviation of 15 fps over a six-shot string. Accurate 40-182H with the same 5 grains of Bullseye in the 10x25 gave 1009 fps, an Sd of 10 fps and shot to the sights at 25 yards. The 40-220H gave 949 fps with a standard deviation of 10 fps and grouped at the top edge of the black on a B15 25-yard timed and rapid-fire pistol target, about right for a 100-yard zero. Both groups very slightly left, maybe one click to center them up.
A dip measure made from a .357 Magnum case measures about 20-21 grains of Goex 3Fg or 4.2-4.5 grains of Trail Boss, filling the case “gently to the shoulder without compression.”
The maximum black powder charge is 24 grains using a drop tube or compression die. About 4.5 grains of Trail Boss is a “full charge” load, with 4.8 grains being the “absolute full stop not to be exceeded,” because Trail Boss should NOT be compressed!
I tried lighter revolver charges with Bullseye and Accurate 40-220H and 40-224H, looking to find a low recoil load of traditional black powder velocity level, approximating the .41 Long Colt. A charge of 3.5 grains of Bullseye in the 10x25 case, with 40-220H gave 760 fps with a standard deviation of only 7 fps and an extreme spread of 29 fps over a 12-shot string. Substituting the 40-224H ogival wadcutter results were almost identical with 748 fps, an Sd of 7 and a 12-round ES of 23 fps., producing satisfying 2-1/2” groups at 25 yds.
I plan to test the above revolver loads in The Infamous Bunny Gun as soon as weather permits.
Overall length of the 10mm Bunny Gun is 38 inches, weight 5 pounds, 1 oz. without scope and 5 lbs., 15 ozs. with Weaver K4 mounted. Backup irons are XS ghost ring rear peep, with M14 front sight clamped onto base machined integral with muzzle. Objective is a low-noise, mild report of subsonic load without needing a can, with energy approximating .45 ACP or .40 S&W to be an adequate 50-yard "poacher's pet."
I expect that the standard "full charge" revolver load with 5 grains of Bullseye, which gets about 1000-1080 fps from the 5" revolver, depending upon bullet weight, should produce about 1200-1300 fps from the 22-inch rifle barrel. This is a good approximation of .38-40 Winchester black powder loads when fired from the Winchester 1873.
Based on experience with firing the .45 ACP and .38 Special in rifles, I expect that my "light" load of 3.5 grains of Bullseye, which produces 750-800 fps in the 5" revolver, depending upon bullet weight, will probably produce about 1000-1080 fps from the rifle. Subsonic, with mild report, but still hitting about as hard as a .40 S&W or .38-40 revolver fired with factory loads from a handgun.
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As I read this, the wheels in my little froggie brain started turning quickly and visions of a S&W N-frame in 10X25 R. That would be able to take any load you would put in the BH, but would have the advantage (IMHO at least) of having a swing-out cylinder for easier loading and unloading and of course the option of DA firing if the situation suggested it. I wish the old Model 28 Highway Patrolman hadn't gotten so sought after... I still remember when they were cheap and plentiful for projects such as this. I guess you might even be able to build an L-frame version of it... isn't there a 10 mm or 40 S&W issue in an L-frame? Oddly enough, with the full underlug barrel, the L-frame might actually end up a little bit heavier than the N-frame(??) 
Originally Posted by Green Frog
