2 part question

[tvc15]

Part 1. Most manuals list a Col for each type of bullet. I assume the is the size they used to test there rounds and achieve the listed velocity. Is this true?

Part 2. If this is true My hornady manual has a col of 3.355 for a 140 grain in .270. this would put the "crimping grooves" out beyond the case. Why put the grooves in the bullet then list rounds that don't use it? Is it OK to load with exposed grooves?


There factory rounds with the same bullet are loaded to the center of the groove and measure 3.280

 thanks, TVC15

[beemanbeme]

While the cannelure is used for crimping, in several bullets it is also a means to keep core and jacket from seperating.  Unless you are loading for a tubular magazine or one of the extra heavy hitters, you can kinda ignore the crimping groove as far a COL is concerned. 

I would guess that the published COL is from the actual ammo they shot.

[Luckyducker]

I believe the COAL listed in data manuals is set according to SAAMI standards for chambers that are cut according to the original specs for the cartridge, but few chambers are the standard length.  This is why I measure my chambers for length and try to load accordingly, although some bullets and firearms have the final say in what works.

[davem270win]

If they list a COL at the start of the data, perhaps with a cartridge drawing, it's probably the Maximum COL as specified by SAAMI. This measurement applies more to the ability of the cartridge to function through the magazine and action than anything to do with the chambering.

If there's a COL listed with each bullet type, then it's probably the actual length they loaded.

When it comes to chamber fit, you generally want to know how far the bullet travels before engaging the rifling. If you seat the bullet out far enough, the bullet will actually engage the rifling before firing, which is supposed to improve accuracy, but if you then try to unchamber the unfired round, the bullet can remain in the bore while you extract the case and powder seperately. Ideally, you want the point on the bullet where the full bore diameter is reached to be just touching the rifling.

My experience is that you need to exceed the MAX OAL before most bullets will touch the rifling.

So, if you're shooting a single shot, the COL can be as long as you want, as the cartridge doesn't need to feed through a magazine. And as already said, unless you want to crimp, the cannelure does not have to at the case mouth.