Linstrum
04-15-2008, 12:59 PM
Monday afternoon I test fired my new .50 BMG “home brew” falling block rifle to check for function and load performance. Testing for accuracy will be later after I make some sights to go on it, the rifle’s basic design is a universal receiver for pressure testing and any optics will have to go on the barrel itself.
The cast boolit I used in my .50 BMG rifle is the simple and basic Lee 500 grain with a diameter of 0.515-inch. It is a non-gas check type designed for the .50-70 U.S. Government and cast in wheel weight alloy. As-cast from my mold blocks it measured 0.516-inches. I machined a piece of AISI 1018 low carbon steel to make a custom sizing die for the Lee Challenger Press (7/8” x 14 turns per inch dies) to size the 0.516-inch down to 0.514” inch, giving two thousandths of an inch all around oversize “bite” for the boolit to seal well without a gas check.
The 30-inch long Freshour rifle barrel slugged at 0.500” bore diameter by 0.510” rifling groove diameter with a turns rate of one in 16-inches.
The powders that I used were 10-grams or 154.3 grains of IMR5010 and the same amount of WC860.
The lubrication used was a thin coating of Johnson’s paste wax applied when the boolit was inserted in the case mouth and then again fresh at the time of chambering the round in the rifle by dipping the nose of the projectile into the semi-liquid wax and then wiping off any drips to leave the two exposed lube grooves full plus a thin but visible tan colored coating of wax on the boolit surface.
I estimated the velocity to be 2000 feet per second by extrapolation of known velocity data taken from chronographed .30-06 cartridge where the ratio and proportion of the number of grains of boolit weight per unit weight of WC860 and IMR5010 powder were the same and yielded 2000 feet per second in the .30-06 rifle. For the .30-06, these powders are too slow by themselves to give optimum performance with 180 grain boolits and I boosted them with about 3 grains of IMR4895 over the flash holes to get maximum energy transfer from the powder to the boolit.
In the .50 BMG with the 154.3-grain WC860 and IMR5010 powder loads, and Lee 500 grain .50-70 Government boolit, I had no leading at all using Johnson's paste wax. I am going to try WC872 next, although I don't expect any noticeable difference in performance compared to the other two powders.
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I got interested in shooting .50 BMG when I was nine years old. I had been out on a desert camping trip near California’s Salton Sea, a huge inland salt water lake that runs just north of the Mexican border to just south of Palm Springs. Early on during World War Two, United States Army General George Patton held maneuvers there to simulate the conditions his army would face in Saharan North Africa when he went in pursuit of German General Irwin Rommel. Patton’s army trained there for many months before shipping overseas and they left a lot junk behind. Back in the days just after World War Two when I was a kid all the debris that was left from the training exercises was still just laying there in the desert, including cartridge brass. I picked up a souvenir cartridge that I still have, it has a 1942 TW head stamp. A year later when I was at Girl Scout camp at Mount Pinos near Fraser Park, California (my mother was a counselor there), I found another .50 BMG cartridge while walking through the pine woods. That one has a 1943 SL head stamp and came from a dogfight a P40 had with a weather balloon that year, the plane having been scrambled from nearby Edwards Army Air Force Base in response to an unknown aircraft spotted from the fire lookout tower high atop 8831 foot-tall MountPinos. For well over half a century I looked longingly at those two empty cartridges before finally doing something about it.
A few years ago I spotted an ad placed by a Florida vendor who was selling brand new surplus Freshour .50 BMG barrel blanks for a really good price. The barrels were 30-inch long 8-groove one turn in 16 inch button swaged rifling with pre-cut chambers and an outside diameter of 1-5/8”. They were made from fully heat-treated and tempered AISI 4140, the legendary chromium molybdenum steel that is among the strongest steels commonly available. Being a machinist, one of those barrels was just perfect for me as the start of my project gun.
The next thing I needed was a receiver. I had already had plans in mind to build a universal falling block receiver that could host any number of different caliber barrels and this was the perfect time to go ahead with that project. I thought long and hard about what the largest caliber barrel would be that I would ever conceivable have a need to test and drew the line at 20mm Vulcan as far as size and strength go but built it for .50 BMG. Back then there were no restrictions on .50 BMG caliber, unlike there is now in a few states. I found a local aerospace alloy vendor who had a block of AISI 4130 steel to make the receiver from, so I bought it and then spent a week machining the universal falling block action and companion universal falling block lock mechanism. When that was done I had the receiver heat treated and tempered for maximum strength, in rough numbers the AISI 4130 being around 239,000 psi yield, nearly equal to AISI 4140’s 250,000 psi and totally adequate for my purpose of making a falling block universal receiver to handle the 63,000 nominal/ 70,000 maximum Copper Units Pressure (C.U.P.) that the 20mm Vulcan and .50 BMG cartridges I designed and built it for. A few days after completing the falling block receiver I heard that California, the location of my new gun at the time, was about to pass a host of laws prohibiting ownership of rifles chambered in .50 BMG. Instead of registering my gun I took it out of California to New Mexico before it became illegal contraband and as a consequence I delayed test firing my gun several years until I FINALLY had time Monday afternoon. I can hardly wait to finish building the scope mount for it and trying it out at 1000 yards!
The cast boolit I used in my .50 BMG rifle is the simple and basic Lee 500 grain with a diameter of 0.515-inch. It is a non-gas check type designed for the .50-70 U.S. Government and cast in wheel weight alloy. As-cast from my mold blocks it measured 0.516-inches. I machined a piece of AISI 1018 low carbon steel to make a custom sizing die for the Lee Challenger Press (7/8” x 14 turns per inch dies) to size the 0.516-inch down to 0.514” inch, giving two thousandths of an inch all around oversize “bite” for the boolit to seal well without a gas check.
The 30-inch long Freshour rifle barrel slugged at 0.500” bore diameter by 0.510” rifling groove diameter with a turns rate of one in 16-inches.
The powders that I used were 10-grams or 154.3 grains of IMR5010 and the same amount of WC860.
The lubrication used was a thin coating of Johnson’s paste wax applied when the boolit was inserted in the case mouth and then again fresh at the time of chambering the round in the rifle by dipping the nose of the projectile into the semi-liquid wax and then wiping off any drips to leave the two exposed lube grooves full plus a thin but visible tan colored coating of wax on the boolit surface.
I estimated the velocity to be 2000 feet per second by extrapolation of known velocity data taken from chronographed .30-06 cartridge where the ratio and proportion of the number of grains of boolit weight per unit weight of WC860 and IMR5010 powder were the same and yielded 2000 feet per second in the .30-06 rifle. For the .30-06, these powders are too slow by themselves to give optimum performance with 180 grain boolits and I boosted them with about 3 grains of IMR4895 over the flash holes to get maximum energy transfer from the powder to the boolit.
In the .50 BMG with the 154.3-grain WC860 and IMR5010 powder loads, and Lee 500 grain .50-70 Government boolit, I had no leading at all using Johnson's paste wax. I am going to try WC872 next, although I don't expect any noticeable difference in performance compared to the other two powders.
~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:~:/+\:
I got interested in shooting .50 BMG when I was nine years old. I had been out on a desert camping trip near California’s Salton Sea, a huge inland salt water lake that runs just north of the Mexican border to just south of Palm Springs. Early on during World War Two, United States Army General George Patton held maneuvers there to simulate the conditions his army would face in Saharan North Africa when he went in pursuit of German General Irwin Rommel. Patton’s army trained there for many months before shipping overseas and they left a lot junk behind. Back in the days just after World War Two when I was a kid all the debris that was left from the training exercises was still just laying there in the desert, including cartridge brass. I picked up a souvenir cartridge that I still have, it has a 1942 TW head stamp. A year later when I was at Girl Scout camp at Mount Pinos near Fraser Park, California (my mother was a counselor there), I found another .50 BMG cartridge while walking through the pine woods. That one has a 1943 SL head stamp and came from a dogfight a P40 had with a weather balloon that year, the plane having been scrambled from nearby Edwards Army Air Force Base in response to an unknown aircraft spotted from the fire lookout tower high atop 8831 foot-tall MountPinos. For well over half a century I looked longingly at those two empty cartridges before finally doing something about it.
A few years ago I spotted an ad placed by a Florida vendor who was selling brand new surplus Freshour .50 BMG barrel blanks for a really good price. The barrels were 30-inch long 8-groove one turn in 16 inch button swaged rifling with pre-cut chambers and an outside diameter of 1-5/8”. They were made from fully heat-treated and tempered AISI 4140, the legendary chromium molybdenum steel that is among the strongest steels commonly available. Being a machinist, one of those barrels was just perfect for me as the start of my project gun.
The next thing I needed was a receiver. I had already had plans in mind to build a universal falling block receiver that could host any number of different caliber barrels and this was the perfect time to go ahead with that project. I thought long and hard about what the largest caliber barrel would be that I would ever conceivable have a need to test and drew the line at 20mm Vulcan as far as size and strength go but built it for .50 BMG. Back then there were no restrictions on .50 BMG caliber, unlike there is now in a few states. I found a local aerospace alloy vendor who had a block of AISI 4130 steel to make the receiver from, so I bought it and then spent a week machining the universal falling block action and companion universal falling block lock mechanism. When that was done I had the receiver heat treated and tempered for maximum strength, in rough numbers the AISI 4130 being around 239,000 psi yield, nearly equal to AISI 4140’s 250,000 psi and totally adequate for my purpose of making a falling block universal receiver to handle the 63,000 nominal/ 70,000 maximum Copper Units Pressure (C.U.P.) that the 20mm Vulcan and .50 BMG cartridges I designed and built it for. A few days after completing the falling block receiver I heard that California, the location of my new gun at the time, was about to pass a host of laws prohibiting ownership of rifles chambered in .50 BMG. Instead of registering my gun I took it out of California to New Mexico before it became illegal contraband and as a consequence I delayed test firing my gun several years until I FINALLY had time Monday afternoon. I can hardly wait to finish building the scope mount for it and trying it out at 1000 yards!