I have just got out my 1851 colt to shoot it again. been many years since I last used it. (late 70's) Low and behold can't use it. The new caps they make now days won't clear the frame. Tried cci and remington. 10's or 11's. Normally takes no. 10 caps. Seems the newer caps have more powder in them and or the water proofing and hold the cap out to far. When you put one on a nipple and try to turn the cylinder the cap hits the edge of the loading notch. I dug through some of my boxes of shooting stuff and finally found a few old caps from the 60's that I used for that gun. They will clear just fine. Original 1851 revolvers had a close clearance between the end of the nipple and the frame back. This insured that no caps can fall off the nipples in use. But now If I were to get them past the frame they would be holding the cylinder ahead and most likely set off a chain fire. The nipples are original but I guess I will have to either seat them deeper or cut them shorter. One other thing is that the flash holes are only .025" dia. Guess those two things are why this gun never had much of any problems with caps coming off or blowing open or pushing the hammer back. Always worked well. shot after shot. Never had a misfire either with it so the holes must be big enough.