AR-15 Chamber Selection, Part Three

In another batch of techno-babble, we look at bullet seating sweet spots in select chambers needed for making better ammunition—and a better performing rifle.

by Joe Carlos

It’s no surprise that every rifle has its own specific sweet spot where it wants ammo to be seated. There is no such thing as a universal seating length or bullet jump. This must be worked out for every rifle built or rebarreled as it is important to group size. My customers receives a free box of ammo with his gun seated to the sweet spot of his barrel. All he has to do is measure the rounds and set his die to duplicate.

In the decade that I was the Armorer for the Army Reserve Marksmanship Program’s Service Rifle Team I gave each shooter a hand press with a competition seating die fixed with Loctite. It was the same press I used in the test shed the day I established his sweet spot. We loaded all of our 80 and 90 grain ammo over length so shooters could use their hand press for the rifle in question and final seat. Idiot proof! Six hundred yard scores for the Team skyrocketed using this approach. Since you don’t have to “chase the throat” (a technique used with VLD bullets to constantly lengthen the seating depth to compensate for erosion) with boat tail bullets, the sweet spot I establish initially works for the life of the gun.

I think a tune up is good at the end of the shooting season to establish a new, longer, seating length but it is optional. If get out to Camp Perry during the CMP Service Rifle phase of the Nationals, stop in my store on Commercial Row (Building 910-A) and visit. I have pictures of test targets showing the value of working out this sweet spot seating length. I’ve seen an average group size reduction of 33% from random seating to sweet spot seating and enjoyed a 50% reduction with the guns I maintained for the Army Reserve. I have worked out sweet spot lengths for hundreds of chambers ranging from the short original SAAMI spec to the long NATO spec. When I compared groups with long range 90 grain Bergers in short and long chambers I couldn’t see any improvement in the long chambers. Those comparisons were all done at the end of the bullet seating sweet spot work ups. Getting bullet seating sweet spot right is more important to tight grouping than the length of the chamber.

Read more in our September 2014 issue. Back issues are available.

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