So a few months back I bought 2000 Oregon Trail Laser-Cast 200 grain RNFP bullets primarily to shoot in a pair of USFA Rodeos in .45 Colt at or below the SAAMI rated pressure of 15,000 CUP. After reading some information on the Missouri Bullet Company web page about Brinell Hardness (BHN) and case pressure, specifically ...
"Lead is a soft metal. Its hardness is expressed on a standard scale, called the Brinell Hardness Number (BHN.) The BHN of the bullet interacts with the pressure generated by the burning powder. The mechanism of this involves the effect of the generation of thousands of pounds per square inch of pressure which causes the base of the bullet to expand, or “obdurate”. Properly obdurated, the base will have expanded beyond its original diameter which has the effect of “sealing the bore” against the explosive pressure of the gases burning behind it. Properly sealed, and working in conjunction with the lubricant in the lube groove, the bullet will thus not allow gases to escape forward from around the base of the bullets, which prevents it from shaving lead from the bullet body and forcing it into the bore grooves (otherwise known as “leading”.)
This failure to obdurate (“seal the bore against onrushing gases”) causes leading which is a chore to clean and is a major obstacle to accuracy.
An optimally hard lead bullet is simply one which obdurates at a given pressure sufficiently to seal the bore against the gases which would otherwise “cut through” the soft lead (called “gas-cutting”, forcing molten lead into your rifling. A bullet which is too hard won't obdurate and seal the bore, because the gas pressure is insufficient to expand the base of the bullet. A bullet which is too soft at a given pressure will experience excessive base expansion and vaporization of the lead, causing leading.
There is a formula for optimal bullet hardness which is simple and it is worth knowing:
Optimum BHN = CUPS/(1422*.90)
The CUPS of your reloads is published in the reloading manuals. Take a typical .45 ACP load, using a 200-grain LSWC bullet – 5.0 grains of Bullseye. This load develops 900 FPS and is in common use among IPSC and IDPA gunners. The reloading manual shows that the pressure generated by this load is 20,000 CUPS. So, the formula for optimal bullet hardness is
20,000 / 1279.8 = 15.62
There it is! For this application – shooting a 200-grain LSWC at 900 FPS requires that you use a bullet with a BHN of 16 to 18 (round upwards a couple of BHN points for flexibility.)
... I called Oregon Trail to find out the BHN of their bullets. I was told that the OT bullets have a BHN of 24!! So if you put 24 into the equation above, the pressure rating for their bullets (to obdurate) is about 30,000 CUP!! If you consider a typical SAA .45 Colt loading from Lyman's 49th edition of 7.4 to 8.3 grains resulting in 928 to 1020 fps and 10,200 to 13,000 CUP respectively, the MBC's equation would indicate that Oregon Trail bullets wouldn't be suited to CAS type loads since at less than 15,000 CUP, they're not even half way to the "optimum" pressure.
So here's the dilemma, given that the OT bullets are on the hard side, I decided to load them for a Ruger Redhawk and Marlin 1894. I made up some hotter loads with 10.5* and 11.0* grains of W231 still under 25,000 CUP (easily safe for the Redhawk and safe for the Marlin) and went to the range. To my horror, the leading in the Ruger was SIGNIFICANT!! I didn't shoot them in the Marlin. So what's going on? Is it that they're still not moving fast enough (not enough pressure) or they're moving too fast at 1250 to 1300 fps. When I called and spoke to a non-technical person at OT today, she told me that their recommended load for W231 results in 787 to 913 fps.
So who's blowing smoke? If OT's bullets are BHN 24 AND MBC's equation is right, how can OT's bullets obdurate in a SAA and how can leading occur in the Ruger.
I'm really frustrated so ANY help would be much appreciated.
Thanks.
** CAUTION: These loads are NOT intended for original SAA revolvers or their clones.
"Lead is a soft metal. Its hardness is expressed on a standard scale, called the Brinell Hardness Number (BHN.) The BHN of the bullet interacts with the pressure generated by the burning powder. The mechanism of this involves the effect of the generation of thousands of pounds per square inch of pressure which causes the base of the bullet to expand, or “obdurate”. Properly obdurated, the base will have expanded beyond its original diameter which has the effect of “sealing the bore” against the explosive pressure of the gases burning behind it. Properly sealed, and working in conjunction with the lubricant in the lube groove, the bullet will thus not allow gases to escape forward from around the base of the bullets, which prevents it from shaving lead from the bullet body and forcing it into the bore grooves (otherwise known as “leading”.)
This failure to obdurate (“seal the bore against onrushing gases”) causes leading which is a chore to clean and is a major obstacle to accuracy.
An optimally hard lead bullet is simply one which obdurates at a given pressure sufficiently to seal the bore against the gases which would otherwise “cut through” the soft lead (called “gas-cutting”, forcing molten lead into your rifling. A bullet which is too hard won't obdurate and seal the bore, because the gas pressure is insufficient to expand the base of the bullet. A bullet which is too soft at a given pressure will experience excessive base expansion and vaporization of the lead, causing leading.
There is a formula for optimal bullet hardness which is simple and it is worth knowing:
Optimum BHN = CUPS/(1422*.90)
The CUPS of your reloads is published in the reloading manuals. Take a typical .45 ACP load, using a 200-grain LSWC bullet – 5.0 grains of Bullseye. This load develops 900 FPS and is in common use among IPSC and IDPA gunners. The reloading manual shows that the pressure generated by this load is 20,000 CUPS. So, the formula for optimal bullet hardness is
20,000 / 1279.8 = 15.62
There it is! For this application – shooting a 200-grain LSWC at 900 FPS requires that you use a bullet with a BHN of 16 to 18 (round upwards a couple of BHN points for flexibility.)
... I called Oregon Trail to find out the BHN of their bullets. I was told that the OT bullets have a BHN of 24!! So if you put 24 into the equation above, the pressure rating for their bullets (to obdurate) is about 30,000 CUP!! If you consider a typical SAA .45 Colt loading from Lyman's 49th edition of 7.4 to 8.3 grains resulting in 928 to 1020 fps and 10,200 to 13,000 CUP respectively, the MBC's equation would indicate that Oregon Trail bullets wouldn't be suited to CAS type loads since at less than 15,000 CUP, they're not even half way to the "optimum" pressure.
So here's the dilemma, given that the OT bullets are on the hard side, I decided to load them for a Ruger Redhawk and Marlin 1894. I made up some hotter loads with 10.5* and 11.0* grains of W231 still under 25,000 CUP (easily safe for the Redhawk and safe for the Marlin) and went to the range. To my horror, the leading in the Ruger was SIGNIFICANT!! I didn't shoot them in the Marlin. So what's going on? Is it that they're still not moving fast enough (not enough pressure) or they're moving too fast at 1250 to 1300 fps. When I called and spoke to a non-technical person at OT today, she told me that their recommended load for W231 results in 787 to 913 fps.
So who's blowing smoke? If OT's bullets are BHN 24 AND MBC's equation is right, how can OT's bullets obdurate in a SAA and how can leading occur in the Ruger.
I'm really frustrated so ANY help would be much appreciated.
Thanks.
** CAUTION: These loads are NOT intended for original SAA revolvers or their clones.