Suppressors Overpressure from Suppressor?

[treillw]

So I did pressure testing to find my max load with my 6.5 CM. 40 grains of varget is overpressure. I backed that down to 38.5 to use as my max load.

Do I need to do pressure testing again when I shoot with a suppressor? I'm thinking it might create more pressure since the can holds the gasses back.

Just want to be safe. Thanks!

 

[jsergee]

No, it will be fine. You should see a slight increase in FPS. By the time the bullet gets to the suppressor the pressure is dropping.

 

[BookHound]

No, it will be fine. You should see a slight increase in FPS. By the time the bullet gets to the suppressor the pressure is dropping.

I very much disagree with this. Suppressors will add some back pressure. Some suppressors add more than others.

The FPS gain is from "freebore boost" which is basically this.... the friction on the bullet from the rifling is gone yet expanding gasses are still trapped within the suppressor tube. Thus, the bullet gets a slight boost in speed. Suppressor manufacturers first really tried hard to eliminate the effects of freebore boost in the early to mid 2000s when the growing consumer market was lead to believe that quality suppressors would yield minimal impact shift. Before then, many shooters enjoyed having some extra velocity by doing nothing more than screwing a suppressor onto the barrel. I mean in today's PRS/NRL world, most guys would love seeing an extra 50-75 fps! In my opinion, having a repeatable POI is more important than any shift from freebore boost. Don't forget we're hanging a weight on the end of the barrel too, which can be another reason for impact shift.

Sorry. I digress....

Modern baffle technology in most quality suppressors does try to eliminate POI shift from freebore boost. It is why there are now lots of baffle designs with relief ports. But suppressors do still add back pressure. It is a byproduct of trapping those gasses. @treillw I strongly recommend that if you want to use a suppressor especially with a max powder (pressure) charge, do your load development WITH the suppressor installed. If you fire the rifle without the suppressor, the load is still safe to use.

Hope this helps. Best regards.