Does anyone make a replacement guide rod assembly for a Sig P320 full size in 45 ACP.

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GrayGuns makes two...

In Super Black finish:
https://GrayGuns.com/product/custom-fat-super-black-guide-rod/

In Stainless Steel:
https://GrayGuns.com/product/custom-fat-stainless-steel-guide-rod/

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True, but they are designed for the 9mm FS. I use one with my P320FS .40, but with different recoil springs.

GrayGuns developed it for 9 mm competition use due to "over springing" on the standard Sig 9 mm RS assembly. It allows a shooter to tailor the recoil characteristics to specific handloads used. They have not tested it with any other caliber.

If you want to use it on any other caliber P320FS, I personally would have no qualms. You have to play with the RS weight and make sure you don't go too light. Others have adapted it to .40, so I see no reason you couldn't play with the .45 as well. Just don't use the 9 mm recoil spring provided. Use Wolff Springs 1911 springs..

Again, it is not officially recommended by GrayGuns for any other calibers. There is NO testing done by GrayGuns for other calibers. There was a thread on this last year, with Bruce qualifying the intended purpose of the unit. Others participated who had worked out spring weights for their own FS .40's. In other words, you are on your own. It is NOT an officially sanctioned replacement for the Sig RSA.

Grayguns is out of the SS guide rods. Grayguns and Top Gun Supply are out, does anyone know some place else that might stock them.

Use the notify by e-mail feature with both outfits. They come in and sell out all the time. Order ASAP when notified.