Does taper crimp reduce bullet diameter?

Just curious, has anyone done testing to determine whether or not taper crimp reduces the diameter of the cast bullet inside the cartridge case?

My specific question relates to the 45-70 caliber bullets cast from one in twenty foundry lead. I have always used a roll crimp but just acquired a Redding profile crimp die for the 45-70 and wondered if anybody had a considered opinion whether bullets cast from this pretty soft mix would be reduced in size within the case using such a combo taper/roll crimp?

It can if you go too far.

I just got the lyman taper crimp die myself and in the instructions it says that if you crimp into the bullet more than .003" it could possibly effect accuracy but dosen't tell you it will defenitly cause problems.

What he said ^^^^^^^^^^^ you measure the case after resizing then only crimp between removing the flare and about .003 to .004" depending on the die manufacturer. I have loaded over 100,000 38/357 rounds using a taper crimp, but these are light target rounds you don't need much k energy for paper.

I think any kind of a crimp can reduce diameter, if it's applied way too heavily.

Test your crimp by loading a dummy round (no powder or primer) then remove the bullet with a kinetic puller. Check the diameter of the bullet before loading and after pulling. Adjust your crimp if needed.

To answer your question, yes excessive crimping will shrink the diameter of the boolit where the case is squeezed into the softer lead.

There is more to this subject than what you are asking. In straight wall cases, that headspace off the case mouth, excessive crimping will reduce the amount of case wall that is available to stop inside the chamber. It is theoretically possible to crimp so much that the cartridge would move far enough into the chamber that the firing pin would not hit the primer. More likely, is that the cartridge would move forward enough that the boolit would not release correctly and the pressures could rise to dangerous levels.

When a boolit is first inserted into the case, the tension holding it in place is the greatest. Anything else done to the case reduces this tension, to a degree. Removing the bell only reduces the tension a small amount. Excessive crimping not only pushes the mouth of the case into the boolit, it also causes some of the case, below the bell, to spring outward. Is it enough to be dangerous, I do not know.

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Originally Posted by Dave W.

...Excessive crimping not only pushes the mouth of the case into the boolit, it also causes some of the case, below the bell, to spring outward...

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That is true of roll crimping, but, not taper crimp. The danger with taper crimp, as stated above, is if done too much the round will slip forward into the throat.

Taper crimps should be just enough to keep the bullet from moving back into the case, ie, not very much. Plunk test your cartridges in your barrel(s) to make sure.

Will it 'size' your bullet? Of course it does, otherwise it would not 'crimp' the bullet. Does it size down the entire bullet? No, a proper taper crimp should only act on the forward section of the bullet and the base end should be full dia.

Do lead bullets get swaged down by the brass? In some cases, yes. If the fired/resized brass ID is not big enough it can reduce the size of the bullet. Depends on the cartridge, case brand, chamber, resize die and dia of the bullet. To 'fix' it you need an 'M' type expanding die (Lyman, NOE, etc) and a properly set crimp die.

Done right, it won't. Done wrong it might.

OF course yes if over done. You can measure it for yourself, just seat a bullet & over crimp. Pull it & measure. SOme calibers are worse than others, some brands if brass are worse than others.