As a catch up note, I have not been ignoring this thread, but not had much computer time the last few days & all of yesterday kept getting "page cannot be found" when trying to log in. Will offer a few thoughts on recent issues & will try to give the rest of my thoughts on the original question later when time permits.
1st; There is one great difference between black & Smokeless propellents, Including 7625. Black powder burns at a very similar rate regardless of confinement, smokeless does not. In the scheme of burning rates black is a fairly slow shotgun propellent, but rather rapid for rifle use. The burning speed of black is due primarily to its make-up with further refinement from its grain size. It is made from very fine "Priming Powder, 4FG up through coarse grain cannon & blasting powders. 2FG & 3FG are the granulations most used for shotguns. Due to its characteristics you could drop 2 drams of 2FG in a primed 12ga shotgun hull, wad it & drop 3/4oz shot & if you shoot it immediately you can just push a thin card on top to keep the shot from rolling out & don't even bother to crimp it. Stick this shell in a gun & fire it & you will get good balistics at a very low pressure. Try to duplicate that with a slow smokeless, including 7625, & you will most likely end up with the shot charge trapped part way up the bore between the wadding. Burn rate of smokeless powders are controlled partially by shape & size of the granulation, but primarily by retardent coatings. The slower the powder, the more retardent & the higher the pressure required to keep it burning effeciently. Fast powders are meant to be used with normal velocities & lighter shot charges, slow powders are meant to be used for higher velocites & heavier shot charges. They should be used for their intended purposes, nuff said for the "Wise".
2nd; "If" I were commisioned to fit up a shotgun having double underbolts & a cross bolt, time & expense not a factor, here's how I would do it. First of course the breech end of the bbls would be fit to come tight against the standing breech with just a wee bit of clearence between their flats & the frame table (bout the thickness of a piece of writing paper). The front surface of the rear bbl lug would be cut on the arc of a circle centered on the hinge & fitted very closely to cross members in the frame bar to assist the hinge pin in carrying the axial load. The lower portion of the front lug below the notch would be shaped as a cam to allow the bolt to come forward on opening the gun bringing top lever back to center & pushing the bolt back in closing, doing away with the need for one of those tedious little ol lever latches to hold it open. The front bolt would be cut straight & given just enough clearence to run freely, no real need for it to actually do any bolting, just the purposes above stated. Rotational bolting would be accomadated by the notch in the rear lug. This could either be cut straight with just bare minimum clearence to allow it to close or cut as a wear compensating wedge. For the former the top lever would always stop centered, while for the later it would start right & rotate around as wear occured. The cross bolt would then be fit to absolutely insure it had a tight bearing on its "Rear" surface to prevent flexing of the breech away from the bbls. If properly fit it would of course also have some bearing on its lower surface to assist the rear bolt it countering rotational movement but this would for the most part be superflous. Utlizing the long bolt of the double underlug system also allows the rear bolt to pierce the bolt having bearing in front of, beside of & behind the lug. As the bolt is pulled up in load this gives it a more equal bearing over its length than if the load is just applied to its End. There are of course many other systems which have been used, many with great success, but this is my take on the very popular double underbolt with top crossbolt system.
Last edited by 2-piper; 08/15/09 07:14 PM.
