What causes this?

[Cnkhunting]

I'm debating building another 280AI due to the two before (mine and my brothers) had problems after shooting would develop a donut in side the case. I loved the round but hated dealing with it. Anyone know the cause?

 

[Black Hat]

I've had this problem with a .243AI years ago.

I put it down to the thicker shoulder brass migrating into the neck after many cycles of sizing and firing.

I can tell you that reaming and trying to push the donut to the outside and turn it off doesn't work, creates more inconsistency than it solves!

Best thing to do is to just size 3/4 of the neck and don't size anywhere near the donut, make sure a projectile will pass though the fire case first however.

How many firings did you have on that brass, I was pushing the envelope, so just ditched it and started to bump sizing and neck sizing when needed, not FLS all the time

If you are not loading the projectile down into the neck shoulder region it shouldn't create any concerns anyway.

 

[Cnkhunting]

It did it right away. I inside neck reamed every loading and it just kept happening.

 

[Black Hat]

It did it right away. I inside neck reamed every loading and it just kept happening.

Interesting, how where you resizing your brass?

 

[Cnkhunting]

I only full length.

 

[cajun]

Hmm. I have a 280 AI with maybe 4 reloads on the brass and no issues. Are you neck sizing or using bushing dies? Edit just saw your post.

 

[Black Hat]

I would not FLS, just bump the shoulder and part size the necks.

You are moving too much brass around when full length sizing, and it has to go somewhere (donut), it can also be the thicker brass in the shoulder moving up into the neck shoulder region.

Being an AI chamber there is already alot brass movement occurring on the first firing, I would do minimal sizing afterwards then anneal the cases.

I would also have your new barrel chamber reamed with a throat long enough to seat the projectile out away from the donut, then you can ignore it.

 

[cajun]

I full length size .002 shoulder bump. Knock on wood no issues.

 

[Black Hat]

I full length size .002 shoulder bump. Knock on wood no issues.

* When I say don't FLS I'm meaning, just bump the shoulder with fls die, as mentioned bu Cajun, don't just send it all the way.

 

[243winxb]

The cause is brass flow. Moves forward, needs trimming. My guess.

Possible fix?

The cutter head on a outside neck turning tool may remove some brass from the shoulder.
Best if the cutter angle matches the shoulder angle.

Like Photo here, with info.

I have not tried a cutter that shaves the shoulder. Just normal outside turning for my 223, 243 & Creedmoor.

 

[MagnumManiac]

280AI has a design flaw, when they sent it off to SAAMI for inclusion, they forgot to subtract the headspace measurement, which causes the donut because the die specs don't have this anomaly.
Brass easily flows from the shoulder into the neck at the juncture and the chamber allows it to continue ad nauseum…

Cheers.

 

[mtbullet1]

try LEE collet die

 

[Seabeeken]

I've loaded 280AI and many other AI cartridges for many years and never an issue. In fact, I have yet to need to trim one. Partly because necks shorten during fire forming but they just dont grow much IME. I only bump .001 - .0015