Donut forming in neck

[gunnut]

My question is. When using Redding full length bushing die for an ar15 The die only sizes the top 2/3 of the neck. Will this form a donut on the inside of the neck. My understanding of the donut is it will affect the burn rate which will affect shot group. If a donut does form will re-sizing with RCBS comp die remove the donut? I have beem loading for my ar whch has a Kreiger upper for 2 years My best group to date is a .177 average is .450 to .500
Russell

 

[thane]

gunnut
What I understand about donuts is this: Every time you fire
a shot, the presure results in a forward creeping of the brass.
Moving in a straight line is not a problem but when the brass
is forced to negotiate a corner it's like a river flow, it takes
time to staighten out. Anyway, the point is everyone has them.
The best thing to do is avoid trying to push a bullet into them
becouse they can couse inconsistant neck tension. If you are using long bullets and you are having to push them in that far,
you may want to modify your chamber for more freebore. Of course now you won't get your loads into the mag anymore becouse they are too long now. Oh crap!, I just remembered
that you can ream inside your neck. I have done this in the past for my .243 win. using the 105's and 107's.
Anyway I hope this helps.
Thane
P.S. There are other factors involved in how donuts are formed
but I didn't want to go on becouse they are only theorys.

 

[gunnut]

Thank you Thane. The info does help & I have considered getting a neck reamer. I shoot 77 sierra,73 berger,77,berger,80 sierra. The 77 siera COL using stoney point guage is 2.330. I seat
my bullets to 2.300. If I understand you correctly you are saying not to seat the bullets past the point that the sizing bushings does not size the neck. If i did this there would not
be much bullet in the neck.
I try to set may bullets up with no air gap in the case. I use 23 grians of varget with COL of 2.300 & a VEL of 2660 with the 77 sierra. Do you find better acuracy with less freebore or a fuul case copacity/no air gap in the case?
Russell

 

[BarryL]

The only time Ive had problems with donuts in my 300 mag and 6.5-284 is when I used rebated boat tails. I think the brass flowed forward and came to a stop against the rebate before the bullet moved. An inside neck reamer solved the problem but the biggest dangers with a donut is the drastic increase in pressure when you seat a bullet forcing the donut to the outside then chamber the mess into a tight neck jamming the dunut back against the bullet. Blew a few primers with a long proven load finding this out.