Headspace with a no-go

[Casualty]

I am finishing up my Savage 450 Bushmaster build, and I'm just looking for confirmation on my headspace situation. I am converting a Savage model 11(110?) From 308 to 450 bm. I have a no-go gauge, but not a go gauge.

Currently, my rifle doesn't close on my no-go gauge. It starts to engage the lug, then stops. I'm guessing 1/4 the normal travel of the bolt handle. It does close on a piece of once fired range pickup brass.

When playing with headspace before tightening the barrel nut, it either fully closed, or stopped where it stops now. I was expecting to be able to slowly dial that in, but it seems to be one or the other.

I forgot that I have to modify my stock to fit the new barrel, so I couldn't assemble, take it outside, and try to close the bolt on a live round.

What says the brain trust? Anything else I should do, or check?

I promise to post a build thread, but I didn't stop for too many pictures. Although, I did my own gun kote(cerakote, but better ), and I'm excited to do more in the future.

[10x25mm]

AR headspace is determined by the barrel chamber, barrel locking collar, and bolt geometries. The barrel retention nut should play no role in determining headspace. It only secures the barrel assembly to the upper.

If the barrel retention nut changed your headspace in any way, you probably have a loose barrel locking collar. This is not a field repairable condition.

[Casualty]

My fault. This is a Savage bolt gun. I will edit the op for clarity.

[Moleman-]

450BM is one of those odd straight walled cartridges that will shrink when fired. Some of the length gets used up when the case expands. You'll get most of it back when you resize. My Manson 450BM GO is 1.700". My NO-GO is 1.709" give or take a .001" as I'm using calipers and not a micrometer. SAAMI is 1.700"GO and 1.710"NO-GO. The fired Hornady brass I have is 1.686"-1.693" where as the factory new unfired cases on cartridges are around 1.696".

.009"-.010" is a decent gap so you're likely fine, but You may want to verify that it will close on live rounds before you get to the range. I don't like how big the difference is between the GO and NO-GO and will likely make a gauge at 1.705" to use as the NO-GO.

[JDG]

Put a piece of scotch tape on the case head of the range brass, see if the bolt closes, I think it's about .003" thick. When you torque the barrel nut, the headspace will get longer too, so remember that.

[Casualty]

Glad I posted. I completely forgot about the tape trick. I'll build the brass up to 1.700 and see what happens.