I have a P226 that will stove pipe on a 20 round mag but runs fine on 15 and 10 round same amno in each mags are Sig not off brand

Disassemble said 20rd magazine.
Clean the tube w/a nylon brush.
Measure the length of the spring.
Stretch the spring 1.5" longer.
Reassemble.
Op's check.

The spring will eventually take to the original set length.
Change the spring.

I'll give it a shot thanks

New Mag? Used Mag? Old school Sig Sauer 'Made in Germany' SWAT Mag? Newer/modern era Mag?

new mag from sig sauer used about 5 times in other weapon no problem there

I would disassemble and clean the magazine. Pay attention to whether or not is was assembled properly in the first place, such that the spring is/was not in correct orientation. Make sure the follower and mag tube/body are clean, dry (no oil) and free of any burrs that could impede function. Then reassemble it properly and check function.

I would NOT 'stretch the spring'. You have what basically sounds like a new magazine there, so if the magazine is the issue, it's either improperly assembled, or has faulty/incorrect components, etc. If that's the case, SIG should take care of you.

Go shooting! Then Report Back...

Also, how many rounds through the P226? Recoil Springs are supposed to be replaced every 5K rounds.
What ammo are you shooting? I'm a proponent of 124 Gr. for maximum reliability/performance.
Does the malfunction occur with a full mag, with just a couple of rounds remaining, or at random?