Sorry ,I didn't make my presentation totally clear either. Your latest answer didn't enlighten me on the land/groove diameter comparisons between .45Cal. M/L rifles and those of .45 cal.Long colts or .45-70s. Are M/L barrels equipped with deeper grooves ,different width ratios of lands/grooves , fewer nos. of lands ,etc. I have given up on the big old slow twist roundball NEF barrels,; not enough meat there! They had been liste on the forum members ' wish-list along with some similar .30 cal ones.The next most preferred item seemed to be a fast twist .45 M/L. I looked at NEF production rifle barrels available in .45 cal. [approx.] that offered a variety of twists ; 1:16 to 1:20. I'm not knowledgable about the ability of a std. rifle barrel's rifling to effectively become a M/L barrel. I felt that a fast-twist M/L barrel might be created if the breech could be bored and threaded to take a 7/8" NEF breechplug.I also have noknowledge of the variety of bullets that would be workable in this caliber Metrics Unlimited is the Mfr. of the plug everyone is rushing to get. Nick Allen? Now to add the feature I've tried to get going from wayback---, what if you took an NEF,I'll start with the 7/8 plug 'cause that's what I have,and have M/U build a 7/8" threaded plug . Then chamber that plug for a .243 short casing~]. The length of that short case would be determined by the dimensions necessary to fabricate the flash chamber ,flash-hole and the powder-pocket. .243 Rems use sm. Rifle primers; .243 Wins. use lg. Rifle primers. With those choices plus 209s, take your pick of ignition sources. Possibly ,when head spacing is accomplished, an ejector notch could be cut in the lower side of the breechplug. To accept the .243 the hex should be larger ,11/16, ground to fit the barrel opening if necessary,at least 1/4 " or 5/16" in height,and use 6 point sockets, ground off square on the lower end to help stabilize the wrench. I have other ideas along this line , but I'm going to hold them until I see how bad you kick my butt on these ideas! That shotgun barrel idea was to use that forcing cone ,you know ,like a SHOTGUN,smoke pole type! Ken