Veral Smith
posted this
02 November 2009
I'm sorry about being so hard to contact for the last couple of months. I had a problem with the email part of my website rejecting all emails, which I didn't learn about for a long time. It is now fixed. My website is LBTMoulds.com. I can be contacted at my email address which is [email protected] or will return all calls to 1 208 267 3588. I'm rarely by the phone, so just leave a message and be sure your phone number is spoken clearly.
I'm quite sure your problem with buckshot groups from your 25-20 is the huge chamber. If a lead bullet tips at any point during takeoff, there is no type of throating that will straighten it out. They MUST be started straight and concentric. In other words, fill that chamber up with the rear part of the bullet. The part inside the case. The ideal body shape for this type gun is for the bullet to have a heavy driving band cut close as possible to groove diameter, with the body as above. It can't tip on take off and will drive tacks if the bore is straight and smooth.
I can and will make you one if you ask for it.
If the bullet doesn't give half inch groups at 50 yards with most any powder charge, lapping the bore, using the LBT lap kit, will make it shoot.
All Marlins I've ever worked with had bores too rough for optimum accuracy with lead, and all are quickly made perfect with lapping. In other words, what I'm recommending is that you lap it before wasting any more time, primers and powder. But don't expect lapping to be the cure all for the undersize molds you now have. The bullet has to fit or it won't shoot.
Here are the mechanics of what is happening now. With low pressure loads your undersized bullets are simply going through the bore tipped sideways. If you get pressures high enough to obturate the bullet base, it will slump out of shape and crooked, then size back down as it enters the rifling, still out of balance and with out of square base.
Oversize cast bullets, unlike oversize jacketed, do not increase pressure. If working at high pressure with cast, which is the only type loads where pressure is a concern, the bullet is going to obturate and fill the chamber 'tighter than Nick's hatband', which just happens to be a bit larger than the bullet diameter I'm recommending.
The worst cause of high pressure with cast bullets is inferior bullet lubricant, which causes the bullets to drag severely while obturation pressure is highest. In other words, the softer the alloy, the more severe obturation pressure against chamber/throat/bore walls will be. An extreme case would be, very soft bullets shot at magnum revolver pressure, when barrel forcing cones are oversize, can obturate so severely that the pressure will split the barrel.
For this 25-20 and it's oversize chamber, plan on making your bullets hard, like water dropped WW, lino or whatever. Ideally 20 bhn +.
Understand that the fat base bullet is going to have to come down to groove diameter, and in so doing will go out of square on the base slightly. Not so badly as to destroy accuracy, but enough that accuracy will never reach the potential of a gun with close chamber dimensions.
If the bullet is started straight and in balance, you won't have any worry about the slow twist not stabilizing them. High twist rates are needed for unbalanced bullets.