Ray, most 618s shoot about 360 to 380 feet per sec and 390 is rather hot , I have heard of a few doing that. Oil the seal ( I use synthetic transmission fluid and have never had a detonation on a 618). Check the breach seal. You may need to cut a new one. I use hole punches from Harbor freight and trim the height when installed and then oil.
These are quite accurate and make good bell target rifles. Great for kids too. Hope this helps. Enjoy.
Ps I rarely take 618 apart. Just oil and shoot.