Need Help with Moly Coated BTHP's

[stonebl]

I am a novice reloader, and I need a little help understanding the impact of moly coating on a bullet and load. I am trying to find load data for a Hornady 75 gr Moly Coated BTHP in .223 Rem, but all I can find is data for a uncoated bullet. What impact will the moly have, and how do I deal with that?

[cdoc42]

Moly reduces the resistance of the barrel to the bullet thus causing a drop in velocity at any given book load. You won't know it unless you chonograph the load. You will then increase the charge to recover that lost velocity, but, again, you need to graph it. Best bet is use the recommended loads and search for accuracy, not velocity, but expect to be able to increase the charge as you move along.

I used to moly all my bullets( .223, .22-250, .270. 7mm Mag, 7mm STW, .300 Win Mag, .338 Mag) but I quit when my 7mm STW lost it's dime-sized accuracy which I believe was due to moly buildup. Moly is also hygroscopic which means it will attract water from the air and thus increase your rust potential. Cleaning your rifle is difficult because, although you may not see copper buildup, your patches will always be black so you can't ever tell how clean your barrel is. I didn't see any increase in accuracy in any caliber.

[c.robertson]

i do not understand that at all.
"Moly reduces resistance of the barrel to the bullet thus causing a DROP in velocity." ???? REALLY??
Guess I have been operating in reverse logic all these years. Reduce friction reduces velocity. Hmmmm. Learn something every day.

[rogn]

Velocity has a close relationship to pressure. When the friction and reistance to passage is reduced the pressure is reduced leading to some reduction in velocity. This seems to hold well with jacketed bullets , cast bullets are a differnt matter, I think related to bullet upset and different pressure curves.

[Brian Pfleuger]

Originally Posted by c.robertson

i do not understand that at all.
"Moly reduces resistance of the barrel to the bullet thus causing a DROP in velocity." ???? REALLY??
Guess I have been operating in reverse logic all these years. Reduce friction reduces velocity. Hmmmm. Learn something every day.

Your logic would be correct if the force pushing the bullet was a constant.

It's incorrect with bullets because the force behind it is variable. One of those variables is the space available for powder burn.

It takes more force to accelerate and maintain acceleration of a non-moly bullet. That extra force means that the powder builds pressure and maintains it. That higher pressure accelerates the bullet to a higher speed.

A moly-coated bullet moves more before the powder can build as much pressure. Burn rate of smokeless powders is heavily pressure dependent. The moly doesn't allow the pressure to build as high, the powder burns slower, the pressure stays lower, the bullet goes slower.