I have the California compliant optics ready M&P15. I'm swapping out the S&W buffer tube with a mil-spec carbine tube. Can I use the same recoil spring? I haven't removed the existing tube yet but it is much longer than the new one I purchased.

Thanks
Russ.

OK, SFAIK there are three lengths of receiver extensions/buffer tubes: rifle, carbine and pistol. The recoil springs have 41-43 coils on a rifle, 37-39 coils on a carbine.

If you're trying to convert a rifle length stock/receiver extension to a carbine system you'll need the carbine tube, recoil spring and buffer-and a castle nut. If you're trying to do a swap between commercial & mil-spec on a carbine, either you ordered or someone shipped the wrong parts. [I've never heard of it, being on the other coast, but could there be a Kalifornistan specific receiver extension?]

OCD purists aside, there's no real need to swap a commercial for a mil-spec receiver extension unless you've got a real Jones for a stock that's only made to mil-spec.

So here's what I've found out.. California carbine models have a commercial buffer tube that is aprox 10 in long. The stock is pined halfway so you can't drive the roll pin out. The inside of the tube is plugged at carbine length so it matches the mil-spec tube I purchased. The buffer spring is mil-spec length.
And yes to reason I'm changing it is because I want a different stock that only comes in mil-spec.

Thanks

EDIT- The spring is carbine length not mil-spec as I stated above.

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THE DIFFERENCES BETWEEN COMMERCIAL AND MIL-SPEC BUFFER TUBES

The significant difference between the mil-spec and commercial AR-15 buffer tubes is the outer diameter of the stock-end, which will also determine the type of buttstock you buy.

At the buttstock end, the commercial buffer tube is .02” wider than the mil-specific buffer tube—1.168" vs. 1.148" diameter respectively. But at the receiver end, the mil-spec tube diameters are .015” wider than commercial spec tubes at 1.185” (including the height of the threads).

The buffer tubes also differ in their manufacturing processes. The mil-spec tube threads—produced from 7075 T6 alloys—are rolled into the metal, resulting in taller and stronger threads. Commercial tube threads, on the other hand, are constructed from 6061 T6 billet aluminum and are cut into the metal, producing slightly shorter and weaker threads but at a lower cost. Other differences vary from manufacturers, including type of metal used and the angle of the back of the tube.

Both carbine length buffer tubes and commercial buffer tubes (AKA by their correct name; Receiver Extension), use a carbine length buffer spring. Regarding length, there is no difference between a commercial buffer spring and a mil spec buffer spring.

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SIC SEMPER TYRANNIS

Given the loads in question either method of forming threads is sufficient for the purpose. Now about depth of threads & engagement, some years back during the semi-annual armorers tear down inspection, I measured the threads on 20 Colt "mil-spec" receiver extensions. Exactly one was blue print specs. Several had ODs smaller than a commercial extension.