I have a pair of 15-22 Performance Centers. They will both put a dozen 60gr Aguila SSS rounds inside a nickle at 25 yards (with some keyhole-ing). I can screw on my 9mm can (9mm because bullet not stabilized) and the gun is silent, except for the sound/flash/smoke coming out the bolt - caused buy the huge increase in momentum from the 60gr bullet.
I'm hoping to get a much heavier JP bolt return spring so the bolt doesn't open prematurely. I realize the twist won't stabilize the pill, but I don't really care as it's so accurate and deadly at 25 yards (and out to 50 yards).
I can't find where to order a JP Enterprises bolt return Spring for my 15-22?
Originally my plan was to install a 1:9 twist barrel one of my ruger 10/22's - but the 10-22 won't feed the 60gr SSS and my 15-22 feed them reliably.

These threads mention the JP springs:

• M&P15-22 Blue Springs
• Blue springs vs JP springs
• blue springs?
• Blue Springs
• Blue Springs Older Serial ???

Really not sure what springs your looking for but here is the Linky to there web site

JP Rifles

Hope this helps....

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M&P15-22’s Tupperware for Guys

The JP springs mentioned in those articles are trigger springs (trigger and hammer). As far as I know, there are no aftermarket bolt return springs for the 15-22.

Aguila SSS opens really hard with that heavy bullet. When I was shooting SSS suppressed through a 15-22 they give a lot more noise coming from the breach than standard bullet weight subsonics. Aguila SSS is loaded with a long bullet in a 22 short case, so the case mouth clears the chamber sooner than would a LR length case. This and the faster bolt speed result in more gas/ flash/ and noise from the breach.

If you want to increase the opening force, swap out your blue hammer spring for a Mil-Spec hammer spring or a Wolf Extra Power hammer spring (orange in color). Your bolt will have to push back harder to cock the hammer. Swap back to your blue hammer spring when shooting regular ammo again or you are likely to have cycling problems.

Thanks for your great explanation.
Edit Looks like that will work fine. Thanks for your help
I actually have two 15-22 PC guns, so I'll keep one dedicated to 60gr SSS.

Range report. I installed the Wolf AR-15/M16 EXTRA-POWER HAMMER SPRING from brownells. Their was only a slight improvement. I was surpised when the gun would fairly relibably work with sub-sonic match ammo (with the original spring it was very reliable, with the EXTRA-POWER HAMMER SPRING, only one or two FTF per mag).
My ALS 50 BMG uses a special made spring that is so stiff a 556 will not cycle it. I could go with that and have a single shot gun - but there must be something in the middle. What about the spring used in my DPMS .308, shouldn't that be have a higher spring constant than a 556 spring?