Excessive Headspace Solved: Peterson Belted Magnum Longs

If you’re shooting new brass in your belted magnum rifle you most likely have grossly excessive shoulder head space. Peterson Cartridge offers long magnum brass that fixes this issue!

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Belted Magnums

We’ve compared belted magnums and non-belted magnums on several occasions. The truth is— the belt is not needed. This does not mean to abandon your belated magnum cartridges like the 7mm Rem Mag and .300 Win Mag, but they do take certain considerations.

Belted magnums have two different headspacing points. The front of the belt is the primary headspace point.

There is also clearance between where the shoulder is on the cartridge and where the shoulder area is in the chamber. This clearance is often excessive and has a negative effect on brass longevity.

If you are blowing the shoulder forward more than necessary with each shot, your cases won’t last as long. This extra clearance also leads to diminished accuracy with brand new cases — whether you are reloading or using factory ammunition. This error can be avoided by fire-forming cases and carefully controlling the shoulder bump.

Below is the SAAMI diagram for .300 Win Mag.

The shoulder is a sloped area that cannot be consistently relied on for measurements due to dimensional variation. The solution is to utilize a headspace comparator to measure where the case goes into the comparator and return the proper base to datum measurement highlighted in the SAAMI diagram.

There are always tolerances with measurements. To achieve minimum clearance, use the minimum chamber and maximum cartridge clearances. For .300 Win Mag, this is 0.0091. Maximum clearance, 0.0261, is found by taking the maximum chamber and minimum cartridge clearances. This is the worst case scenario.

When I reviewed the SAAMI diagram for 7mm Rem Mag, I discovered that the minimum and maximum clearances were the exact same as for .300 Win Mag.

Peterson Long Magnums

It’s important to remember that there is always going to be variation in your clearance and there will always be excessive clearance.

Peterson Cartridge long magnum cases offer a solution to this problem, with shoulders that are pushed forward (longer base-to-datum) compared to SAAMI specs.

They will still chamber in a SAAMI-spec rifle chamber, but will take up the unnecessary space.

I compared some virgin standard magnum brass (Federal and Lapua) to virgin Peterson Long brass. Both were brand new. The Peterson long brass had the shoulder 0.0100” forward than the Federal 7mm Rem Mag and 0.0095” forward than Lapua .300 Win Mag.

This means that the cases stretch that amount less upon first firing! This is beneficial for case life, consistent case capacity, and precision.

Reloading

I’ve loaded five 7mm Rem Mag rounds and five .300 Win Mag rounds in Peterson long brass thus far.

I used RCBS full-length dies on an RCBS Rock Chucker Supreme to resize the cases and seat the bullets with a ChargeMaster Link to charge the cases.

I loaded Ramshot Grand for both calibers, using a Berger 7mm 180 Grain Hybrid Target Bullet for the 7mm Rem Mag and the new Sierra .30 Caliber 200 Grain MatchKing X for the .300 Win Mag.

The Test

I have two near-identical rifles set up in 7mm Rem Mag and .300 Win Mag. Both are Bergara B-14 Wilderness HMRs. I added ARCA rails to both and fired all five rounds.

The next step was to compare the fired brass to unfired brass. The 7mm Rem Mag Peterson long brass only stretched 0.0050”.

This would be 0.0150” if compared to regular brass! The shoulder only blew forward 0.0065” for the .300 Win Mag Peterson long brass. This would be 0.0160” compared to regular brass!

After fire-forming the brass for the first time, you can control how much you want to resize the brass.

Conclusion

If you’re looking to get the greatest precision possible out of your 7mm Rem Mag and .300 Win Mag loads without sacrificing case life, be sure to try Peterson’s long brass.