Author Topic: 224 SAAMI Chamber Spec  (Read 204 times)

Offline carnaby

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224 SAAMI Chamber Spec
« on: April 16, 2019, 07:16:18 PM »
This summer we'll have 24" 224 barrels with new specs. We're going to 4-groove 1:6.5 twist rifling and our new chamber will have a 0.255 neck diameter. We chose 4-groove so that our gas port cannot take out an entire land.

I decided to have JGS make our new chamber reamer. I sent them dummy rounds with the new 90 SMK bullets from Sierra with the thicker jackets that Federal requested. Sierra was kind enough to send me some samples for this purpose. The dummy rounds were seated to give a COAL of 2.285" and we requested a bullet jump of 20 thou with this COAL because we want hand-loaders to be able to load to mag length to use all available case capacity with ammunition comfortably loaded to magazine length but without excessive bullet jump. JGS measured the rounds with an optical comparator in order to design the new reamer, and when they were finished, they sent me the print.

Now things become interesting... the print showed freebore of 0.025", which is 0.025" LESS than SAAMI min specification. This is clearly not desirable as I don't want freebore less than SAAMI min spec - otherwise, I would most definitely have an out of spec reamer.

Puzzling. I read on this forum that a 90 SMK bullet jump of 2 thou is expected for a SAAMI spec chamber with a COAL of 2.276". I'm also told that JGS is the gold standard for chamber reamers. But with a JGS reamer that is spec, according to JGS, except for the neck diameter reduced from 0.258 to 0.255 (which does not change the bullet jump to the lands), they tell me I will get bullet jump to the lands with a 2.285" 90 SMK round of 0.045", or an OAL to the lands of 2.330".

My current barrels made with a PTG reamer exhibit OAL touching the lands of 2.337" and this is supposedly out of spec?