Of recently, I just got a bit curious on how a person can determine that the .223/5.56 rounds are really indeed good to go. I recall a while ago I went to this range. The range allowed people to bring their own ammunition, but prior to it they checked it with some sort of gauge. The gauge looked similar to that of a iron. What they did was put the ammunition on it, and somehow they knew that it was safe to fire. If someone brought in some ammo that they couldn't read with the gauge, they told them they could not fire it. Curious, I asked what it was but never picked up the name of the device they used.

So my question is, what sort of device/gauge can you use to check ammunition, like .223/5.56 rounds, to determine if it will fit and fire properly (minus of course any other issues outside the scope of proper case head space). I know there are headspace gauges themselves, and I have the go/nogo/field gauges handy that I check from time to time. However, I am not sure how to verify/check actual rounds to verify that they actually fit in a proper spec chamber.

I bought some Mo Defina gauges, and I been playing with it for a long time now and can't quite understand how to determine if a round is good to fire. Did I waste money or is there any way to verify that prior to using the round, that it is in the proper dimension ?

Again, this is mostly for curiosity reasons and I don't use reloads or plan to reload on my own. So I am wondering with factory ammo, is there some way to verify that the ammunition will fit and fire properly in a .223/5.56 barrel?

Are you sure that they weren't checking for steel core ammo?

Is that what it is? Where they put a magnet on it to see if it sticks? I'm not sure, just got me really curious. The guy explained it but he was really busy acting so he sort of mumbled it and I didn't catch a single word of it.

That would be my guess.  I've never heard of a range checking ammo for anything other than that.

Yep. Some ranges don't allow bimetal jackets or steel core.

A Mo's gauge will check the cartridge headspace. When you shoot the cartridge, the case blows out to fit the chamber. You take the fired case and run it into the gauge and note the reading. When you resize the case, you need to knock it back below the fired reading. It's not something you test factory ammo with unless there is some odd functioning issue.  

If you put an actual chamber headspace GO gauge in the Mo's gauge, it should read 0.

B

Ah let me try that tonight. Thanks for the answer. If the go gauge reads 0 could I assume that factory ammo should read 0 as well? And use that as a basis to test factory ammo? Or is that a crazy idea?

Factory ammo should be good to go. If you have go/no go gauges then drop the go gauge in - if it closes you're good. Drop the no-go in, if it doesn't close you're good.

Checking every round of factory ammo will be VERY tedious and pretty much a waste of time. Reloads I could understand, but quality factory ammo there's no reason.

Near 100% chance this was simply a magnet as most indoor ranges have policies against bi-metal jacketed projectiles....mostly just because they really just don't want steel cases mixed in with all the brass casings they sell.

I think you're misunderstanding what most mean by GTG.  First, the guy at the range was likely checking with a magnet - many ranges only allow non-magnetic ammo.  As for shooter opinion on GTG, I shoot it.  If it runs my AR properly (no short-stroking et al), and prints an acceptable group (1/2" at 300 yds...just kidding, but I'd still like to see <2MOA for low-precision blasting and well less than 1MOA for any kind of accuracy), and I had no misfires, no split or torn cases, no popped primers or other signs of overpressure, or anything else that would be considered bad, then I'd call it GTG and recommend it.

Hey, I tried the GO gauge and dropped it into the Mo's gauge. It doesn't quite read 0 but dang near. The GO gauge is a Clymer 5.56 measuring at 1.4636. I tried the .308 and others and they have the same results in the gauges.

FPNI

I'm sure it read zero on Mo's Gunsmith's headspace gauge. Check out all the different dimensions on a GO HS gauge.

Thanks. After playing with this Mo gauge last night, I'm fairly convinced I got it figured out. I cross compare the specs at SAAMI. The Mo gauge measures from .00X of the mentioned spec. So if the gauge reads 2 that means .002 from the spec.

First, the .308. I'm fairly convinced that the Mo's gauge starts at 1.629. The 1.630 GO gauge for a .308 round reads +1 so it makes sense. To verify this, a field gauge is measured at 1.638, so the Mo gauge should read 9, which it does since 1.629 + .009 = 1.638.

So for a Mo gauge to read in .308 fine, it's suppose to be between 0-1 for it to be safe to fire.

Second, the .223. From the SAAMI spec, the minimum head space is 1.4636. I'm fairly convinced my 5.56 gauge from Clymer gauge is measured at 1.4646, at least according to your site for the 5.56 GO gauge. So the Mo gauge should read .001, or 1 and then a tad bit more than half way between 1 and 2, which it does. Now to test this theory, I used a .223 FIELD gauge measured at 1.4736 which should measure at 10 on the mo gauge for .010 from the 1.4636, which it does.

So for a Mo gauge to read in a .223 fine, it's suppose to be between 0-1.6 for a .223 and 0-1.6 for a 5.56.

Of interesting note, the snap caps are SHORTER than the SAAMI spec. As much as -.005 to -.008 from the GO spec line mentioned above. That's a pretty major difference I assume for reasons as to not slam into the chamber and potentially ruin it during training. This is the main reason why NOBODY should use a snap cap as a GO gauge.

A Mo's gauge is exactly like a RCBS Precision Mic. It's use is for setting up reloading dies and adjusting the amount of sizing. It's not really for checking if ammo is safe to fire.
Don't make the assumption the chamber is at minimum headspace condition. It's really a tool for relative measurement.  

Typically what you do is take a couple of fired cases and check them in the Mo's gauge. After running them through a resizing die, they should measure shorter than the fired length. Again it's a relative measurement, but they do use HS gauges to make them.

B

According to the paper it came with, it can be used as a headspace check. It says: "The combo gauge for the rimless cases measures both case length and head space."

But yeah you're right. It doesn't tell that it is safe to fire since there are other factors. However, for commercial ammo or even reloads that you buy, it appears like you can drop it in the gauge and spot check a few rounds to determine if it is too long or too short. According to a few blogs, it would appear that match shooters spot check their rounds prior to matches to determine if the headspace is too long for their rifle.

It has a number to Mo in Brookfield, CT. I'm going to try and give the guy a call to ask him.

I know people like thinkin that but that's mostly not true. Steel-cased ammo mostly comes loaded with bi-metal bullets. While they don't damage the range's backstop, they can cause sparks. In indoor ranges, unburnt powder accumulates forward of the firing line. Sparks can ignite that powder and start a fire that burns down the building. Of course, actual AP ammo can damage the backstops. That's why indoor ranges check for it.