Learn from my dumb mistakes! How Not to Use GO-NO-GO Guages

[AregularGuy]

Ugh! Building my first rifle in probably 4 years. It happens to be a .308 semi auto. You may have seen my other thread about a stripped handguard screw. That happened when I forgot to install the muzzle brake and wanted to take the handguard off to use my barrel block.

It's been quite a while since I checked headspace on a rifle and it seems I forgot how to properly use the gauges. The rifle was fully assempled, upper attached to lower. I stripped the bolt, removed the extractor. Inserted GO gauge into the rifle, gently closed the bolt (yes, I know that you are already palming your face).

Now, the bolt and barrel are brand new and I will admit a tight fit. So I dropped the bolt from about 1-2" open. Closed bolt, so far so good. Next to the NO GO gauge. Inserted the gauge gently closed the bolt. Bolt was open a bit so for good measure I dropped it from 2-3" open. Didn't close all the way. Success! Except, when I tried to remove the gauge the BCG was stuck, really stuck. I had to oil the crap out of it, then bounce it on the stock while pulling the charging handle. Eventually it slowly worked itself unlocked. Then I remembered I should have disassembled upper/lower then checked headspace, or better yet, check before assembling upper. Doh!


TLDR: Don't use GO-NO GO gauges on fully assembled rifle!

[baih777]

You can use the guages on a fully assembled ar. Just dont let the bolt go.
Gently ride the bolt into position.

I usually check the headspace before i install the barrel.

All you needed to do was use a cleaning rod and tap the guage loose.

[AregularGuy]

You can use the guages on a fully assembled ar. Just dont let the bolt go.
Gently ride the bolt into position.

I usually check the headspace before i install the barrel.

All you needed to do was use a cleaning rod and tap the guage loose.

Yes, correct, don't let the bolt go, which I did. I also tried the cleaning rod, tap, tap, even while pulling down on charging handle. That was a no go and ruined the cleaning rod. It was stuck good.

ETA: It was not the gauge that was stuck it was the bolt that rotated and locked/jammed into position out of battery. It was already a tight fit and I added to the issue.

[subscriber]

The locking lug edges have a slight ramp or chamfer on their mating corners. If you drop the bolt those little ramps try to cam the bolt shut, such that it tries to compress the gauge. Then you could end up with a jam... Then the question becomes, are either the bolt or barrel extension lugs dinged? Is there a ding in the chamber? Unlikely, but you should take a look.

Gauges are distance measurement devices. Not force measurement devices. You don't use force with a go or no go gauge. That includes dropping the bolt. That is like torqueing a micrometer shut to get a smaller reading; then wondering why it no longer reads zero when you close it with nothing between anvils...

If you are going to remove the extractor, you should also remove the ejector. Else the ejector spring is pushing against you closing the bolt, and messes with getting the feel that reports gauge clearance / interference.

[alpha_romeo_XV]

YouTube doesn't always have what you're looking for - but in this case it does.

[AregularGuy]

YouTube doesn't always have what you're looking for - but in this case it does.

Yes, I should have double checked the method. I've done this many times before but not for 4 years or so. Seems I forgot more than I reckoned.

[sigstroker]

Why don't gauge manufacturers trim off the "rim" so you don't have to remove the extractor and ejector? That's what the SOTAR guy did to his gauges.