Colt buffer assembly, left, and spacer fitted to CAR action spring, right
The opposite end of the carbine action spring fit the bolt shaft perfectly. The overall diameter of the spacer allowed it to slide down the CAR-length buffer tube. However, it fit tight enough to not allow side-to-side rattles. However, I had to cut the overall length down to .906 so the bolt stop would engage.
Hahn Precision sub caliber buffer assembly, 8 oz.--note the longer full-diameter section that increases action spring tension.