We aren't talking about any additional forces on the valve, hammer, or rear cap.... we are talking about the additional force on the bolt itself.... The bolt lockup in an MRod is achieved by the cocking pin under the bolt.... It pulls the hammer back to cock the gun, then when you close the bolt it slides forward and rotates left or right (depending on which way you have the bolt handle set up) into a slot in the receiver.... That pin in that slot takes ALL the force on the bolt when you fire the gun.... At 3000 psi, that's only 83 lbs. in .177 cal, but it's 167 lbs. in .25 cal, and 330 lbs. in .357.... It could be as much as 450 lbs. in .408 cal (depending on the O-ring and bolt size sealing the breech).... all trying to shear off that single 8-32 screw that is threaded into the bolt.... If that pin shears off, there is nothing to prevent the bolt from flying out of the back of the breech and across the room.... except whatever part of your body might be in the way, of course.... If you increase the caliber above Crosman's .25 cal, IMO it is prudent to change that 8-32 screw to a 10-32, set deep enough into the bolt that the shear plane is not through the threads of the screw.... Even worse is what could happen if you forgot to push the bolt handle down, or it got knocked back up, and then you fired the gun.... Instead of the cocking/lockup pinn only having to withstand a stationary force, now it gets a running start, giving the mass of the bolt energy and momentum, until the nose of the bolt pops out of the back of the barrel O-ring.... You would be astounded as to the destructive power that energy has.... Lloyd did an experiment a while back where he sheared a 1/4" bolt handle clean off under those circumstances.... Any increase in pressure above Crosman's 3000 psi fill increases the loads under all conditions, of course.... in direct proportion to the pressure....Bob
I'm not sure I follow your question about how the Cothran valve would work in a stock MRod.... You will have to change the transfer port, a stock one won't fit, and if you leave the stock breech and barrel porting I would ask why would you bother installing a Cothran valve in the first place?.... Regarding the performance of the Cothran valve in an MRod in .308 cal, Lloyd covered that early in this thread, I believe it was in the 200 FPE region.... Lloyd's summary is in reply #161 on page 9.... Assuming you use 1/4" porting in the transfer port and barrel to take advantage of the Cothran port sizes, and a 24" barrel, I would think in the 150-160 FPE region with 100 gr. bullets would be feasible at 3000 psi, with each subsequent shot losing a bit of velocity.... The velocity of the Cothran valve appears to be set by the pressure it has available for that shot.... I personally would increase the cocking/lockup pin to 10-32 in a .30 cal, but I know many have built .30 cals and I have never seen that mentioned, so presumably nobody has had an accident.... I would advise you to make sure the bolt is locked in any MRod before you pull the trigger, and the bigger the caliber, and/or the higher the pressure, the more critical that will become....Bob
Since I am not equipped to make little pieces of metal from larger ones I opted to send all the Marauder parts to someone with more skills and knowledge than me and with the necessary tools to get it done right. The only metal working tools I have are a hacksaw and Dremel.
My question was more around the what would you expect out of a 25 caliber barrel sitting on the gun as is.
QuoteMy question was more around the what would you expect out of a 25 caliber barrel sitting on the gun as is.If you only dropped in a Cothran valve, and assuming you could get a transfer port to seal up to the original barrel, if you did not modify the barrel port you may gain very little, if anything.... The stock barrel port will severely restrict the power.... Not having done this, I can't give you a number....Bob
So you are NOT talking a stock MRod, you are talking about enlarging the barrel port to match one of those that Don supplies.... I would think at 3000 psi you might get about 80 FPE with the 0.160" port and 90 FPE with the 0.180" port, but that could vary significantly depending on pellet weight.... It may be less because of the MRod's 20" barrel.... These numbers are really not much more than an educated guess.... Bob
I believe on the 177s and 22s its .110 and the 25 is .140 stock but I could be wrong.Mike
Gen 1 guns in .177 & .22 were a tight .125" The .25 valve was .140 ALL the Gen 2 calibers are at .140" from what i've made notes on anyways
Baker airguns, formerly B&A, sells a big bore breech for the marauder. Barrel bore is at 1/2" dia. Ad said it will handle calibers up to .357. It's under $70. Not sure if this would help in this instance.
All other things being equal, as a very rough estimate, the FPE is proportional to the transfer port diameter, up until the point where the port equals the caliber, which is the maximum.... This assumes, of course, that the port you are measuring is the most restricted point in the system.... MRods are sometimes restricted by the diameter of the bolt probe as well.... Bob
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