Help explain taper crimp

[SteelBanger]

So I'm loading 350 Legend for the first time, using the Hornady 4 die set, virgin Starline brass, and 125gr hammers ... but first I loaded up some flat base 124gr RMR bullets (just for plinking around) and I had to bell the case mouth about 3 thou to seat them. I still haven't shot these but I prepped the cases for the hammers the same way.

The case mouth ID after sizing is about .353 and I'm stuffing a .355 bullet into it. Adding a 3 thou bell to the case mouth gets me to .356 ID and it's just enough to seat the RMR bullets and of course the hammers seat just fine with the boat tail.

My first question is since the hammers have such a pronounced boat tail, is belling the case mouth even necessary and if not, is the taper crimp needed if there's no belling? I'm seating the hammers to the middle of the top ring / groove if that matters.

Second question is on applying the taper crimp to the RMR bullets. After seating the bullets my case mouth is still .356 but the bullet base pushes the case out to .356 for the length of the bullet, essentially eliminating the bell. I've set up the TC die per the instructions but the case mouth stays at .356. How do I know if I've put too much or too little TC on it and is it even needed if I have that consistent tension on the entire bullet base?

 

[MagnumManiac]

I do not know how this round headspaces, but on a round that headspace on the case mouth, like 38 Super and 45 Auto, the taper crimp squeezes the case into the mouth more so than the body, allowing positive headspace. A roll crimp would exaggerate the headspace making it longer than it should be by curling the mouth into the crimp cannelure/groove.
A boat tail bullet doesn't need a belled case to seat into, it really shouldn't need crimping either, but a light crimp may be necessary depending on how much neck tension there is initially.
As you increase a taper crimp, you should see a reduction in diameter at the mouth by around .001", that is all it will give you.

Cheers.

 

[SteelBanger]

Ok thank you, this cartridge headspaces off the case mouth just like a straight wall pistol cartridge so a roll crimp is definitely a bad idea. I think I sort of answered my own question above but I was probably over thinking things. I only had 20 cases prepped with the bell on them for the Hammer bullets but moving forward I'll skip the belling for those bullets.

And even with the RMR flat base bullets they have just the slightest rounded edge on the bottom so I'm able to bell to the bullet diameter of .356 and get them to seat ... then as the bullet base seats into the case it pushes the case diameter (ID) out to that same .356 for the length of the bullet, which effectively takes out the bell and gives me about 3 thou of neck tension. The loaded rounds chamber just fine so I think I'm good here and adding some taper crimp isn't changing the OD of the case mouth, in fact I think it'd have to squish the bullet to do so.

 

[Lefty7mmstw]

Same as a revolver or auto, if your bullets are walking, you need to add more crimp, otherwise you are probably fine for your rifle. I only really use a taper crimp on 40 s&w right now, but it is really to get rid of the bell and get the neck tension up. Uniforming the neck diameter is a bonus but it really shouldn't help your headspacing as long as the rounds are going into battery; there is a bit of a shelf at the front of the chamber that the mouth catches for headspace.
Over crimping can actually cause the mouth to start to slip the shelf and cause inconsistent headspace. This really is not much of an issue in most straight walled rimless but has been an issue in 38 Super and possibly a few others.

 

[bpcrshooter]

what rifle is it in?? single shot rifles dont need much crimp, even bolt actions do not need a lot. Now if its tube fed your going to need some crimp to hold it in place so it doesnt push in.

If it were me, I would re-size and not bell/expand, I would just seat he bullet and try it w/o crimp first seeing as you have BT bulets. If it looks as tho the bullets are moving add crimp til it stops

 

[SteelBanger]

It's in an AR platform, but legal restrictions require it to be single fed with a bob sled for deer hunting in my state. So to your point no I don't need to worry about other loaded cartridges in a magazine having any bullets moving back. Now, I do have full capacity mags for it and will shoot it as a semi-auto for plinking purposes with those 124gr RMR bullets, but I think the 3 thou of neck tension is likely enough. I'll do some testing and check for seating depth changes and adjust the crimp as needed.

Thanks guys!