Measuring for a form one

[gfercaks33]

how do you get the measurements, and how exact do you have to be.

I was going to buy a prebuilt with an 11.5" barrel and it says it's overall length is 20.5" but I think that's including a2 birdcage. Stock will be a magpul str it say it's length of pull is 11.10" - 14.35".

Any tips on getting the right number.

 

[gfercaks33]

30.5" is the general number I'm seeing for 11.5" barreled ars.

 

[CAR-AR-M16]

My M16 with a 12.5" barrel has an overall length of 32". Length is taken with stock fully extended and muzzle device does not count unless it is permanently attached. Lengths do not have to be super exact. Within a 1/2 inch or so should be fine.

 

[mr ed]

I would make it the maximum you would potentially use it in the future. I don't know about sbr's but years ago when BATF inspected my inventory.
I asked about a thread adapter for an old suppressor I had and their response was permanently attach adapters to the various guns. Because if they measured the suppressor with the adapter attached it would put it over the maximum length listed on my paperwork and they would confiscate it. They also said it could be shorter, such as getting damage to the tube and having to shorten and re-thread. just can't be longer.

If the same mentality applies to sbr's and you paper it as say a 7" barrel with 25" overall then next year you put on a 12" barrel it would exceed your overall length.
Measure with stock fully extended and add an inch to the longest barrel you might possibly use to be safe.