RCBS precision mic and headspace help

[GreggNY]

I just finished assembling everything I need to start reloading and am working on figuring out shoulder bump/headspace measuring. The rifle is a 700 5R .308 and I've been shooting FGMM 168 and 175 exclusively. I've been using the RCBS precision mic to compare fired vs unfired FGMM cases and have questions. Basically all my fired brass is measuring at 2.5 thousandths below 0 on the gauge. The unfired 168gr measure 3.5 thousandths under 0 and the unfired 175gr measure 6 thousandths below 0. I measured a good 10+ cases of each and they are all very similar. My rifle shoots the 168s VERY well, and noticeably better than the 175s.

So how far should I bump the shoulder on my brass to reload? I've read that I shouldn't bump the fired cases all the way back to factory unfired length, but the 168 unfired length is only 1 thousandth smaller. Any advice?

Fired brass:


168gr unfired:


175gr unfired:

 

[pmclaine]

Measure 10 cases fired from your rifle using the precision mic and get an average of its reading. Sounds like you did this. Doesnt matter what the average is it might be +.001 or -.003. It is what it is and represents your chamber size.

Whatever that average measurement is I now consider that "0".

From "0" bump back .001-.002 for bolt rifles and .003 to .005 for semi auto.

The zero on theRCBS mic is supposed to mean something but not in relation to your fire formed brass.

The Precision mic works but I didnt like how it wont give me a hard number. I moved on to the Hornady headspace gauge because it gives me hard repeatable numbers so I know exactly what my size die is doing.

The Rcbs tool will work, it just wont give you hard data.

 

[GreggNY]

Measure your 10 cases using the prcision mic and get an average of its reading. Sounds like you did this.

Whatever that average measurement is I now consider that "0".

From zero bump back .001-.002 for bolt rifles and .003 to .005 for semi auto.

The zero on theRCBS mic is supposed to mean something but not in relation to my brass.

The Precision mic works but I didnt like how it wont give me a hard number. I moved on to the Hornady headspace gauge because it gives me hard repeatable numbers so I know exactly what my size die is doing.

The Rcbs tool will work just wont give you hard data.

Thanks for the reply. This is pretty much what I did. If you consider the fired cases at 2.5 thou below the 0 mark on the scale as the ZERO starting point, the unfired 168gr FGMM are only .001 smaller. So should I even bump the shoulders at all? The advice you gave is the standard I've seen pretty much everywhere and is what I was expecting to do. This is until I saw how small the difference was between my fired and unfired cases. Now I'm stumped.

 

[pmclaine]

see my edits above. cleaned it up a little.

 

[roggom]

The zero on the RCBS mic should be 1.630-1.640 for 308, although most factory ammo will read a "few" thou under. As PMC stated, for bolt, i would bump it back .001 to .002, so -.004 on your mic.

I went and measures some unfired 168 FGMM and all of mine were .001-.002 under the zero on my mic. So your mic may be a little off, but as long as it is consistent you should be fine.


Remember brass will fire form to fit your chamber and then spring back a bit. So as you full size the case it will grow a bit, as firing will shape the brass forward to meet the chamber shoulder and outward to fill the chamber. Just dont push it back too far as you can end up with a breakdown at the case web or fail to fires.

 

[pmclaine]

I try to get as close to -.003 (I load for semi auto) from fire formed size this limits overworking the brass and should ensure reliable feeding. I note some brass gets squeezed more or less depernding on lube, my handle operation or something with the brass and even those sized at -.002 will feed in my guns but maybe not others.

You are doing your best and thats all you can really hope for. Your technique is much better than "just screw the die down to the shell plate plus an 1/8th turn".

 

[mdmp5]

Bump the shoulder on your fire formed brass anywhere from 0 to .002 back. Keep in mind that even though if you set your die to do this, it won't bump every single piece of brass the same. Some will be longer, some shorter. Some brass may actually appear to grow; the shoulder bumps forward. If this happens, it means that the brass shoulder is already pushed back further than the dimension of the die. So the body gets sized, and the brass squeezes forward to fill the dimension of the die, pushing the shoulder forward. Each piece has different rigidity/spring back, so expect variance. As long as you size the body, the brass should chamber. If not, turn the die down until the majority of your cases start having shoulder setback.

I recommend using a cam over with your press so you get the most consistent results. But if you cam over too much, you will be bumping further than desired. Start with a light cam over and see how that works. If the brass is bumping too much, either use a redding comp shell holder with the next size down, or unscrew the die in 5 degree increments (5 degrees = .001). If not enough, use a comp shell holder with the next size up, or screw in the die in 5 degree increments.

 

[MaxLoad]

Thanks for the reply. This is pretty much what I did. If you consider the fired cases at 2.5 thou below the 0 mark on the scale as the ZERO starting point, the unfired 168gr FGMM are only .001 smaller. So should I even bump the shoulders at all? The advice you gave is the standard I've seen pretty much everywhere and is what I was expecting to do. This is until I saw how small the difference was between my fired and unfired cases. Now I'm stumped.

Forget about the unfired cases, they have zero relevance in what you are trying to achieve...

Only measure the fired cases, and setup the die accordingly.

Best results come from brass which have been fire-formed multiple times. I generally neck size and fire 2-3 times and keep those cases as reference cases. Generally they ALL zero exactly at the same point. I then bump .001 from a well fire formed case.

 

[GreggNY]

Forget about the unfired cases, they have zero relevance in what you are trying to achieve...

Only measure the fired cases, and setup the die accordingly.

I guess this is it in a nutshell. The fired cases are now exact to MY chamber, and just size back .001-.002 so they will chamber again. Correct?

What confused me was thinking that all factory ammo out of the box was sized to saami specs to chamber any rifle, and in bumping the shoulder, I didn't want to take the case all the way back to that short of a length. Since there are obviously variations in factory ammo cases as I found in my unfired 168 vs 175 FGMM, the starting point unfired length doesn't matter too much. Correct again?

Thanks again for the help guys

 

[bigedp51]

I have the same RCBS precision Mic and "MUCH" prefer the Hornady cartridge case headspace gauge because it will work with all calibers and tell more. The one reason I say this is because the RCBS precision Mic has a hole in the bottom of the gauge to prevent the primer from interfering with your readings. And sometimes knowing how far the primer is protruding from the rear of the case can tell you far more than you thought it did.

As stated above only your fired and resized case lengths are all that matters, "BUT" knowing your chamber actual headspace length helps.

I'm hoping you have a set of vernier calipers for this test.

Resize a case to the shoulder bump setting you think is the proper shoulder bump length. (minus .001 to .002 in a bolt action)

Now measure the case and write this measurement down.

Now insert a fired spent primer in the primer pocket with just your fingers starting the primer into the primer pocket.

Now slowly chamber this test cartridge closing the bolt and letting the bolt face seat the primer and then remove it.

Now measure the case again from the primer to case mouth and write it down.

Then subtract the first case measurement from the second and this reading will be your head clearance measurement.

Congratulations, you just made the cheap bastards headspace gauge.

Now how far off was your shoulder bump estimate from your head clearance measurement, and which one do you think is more accurate.

 

[roggom]

Now that is pretty slick ^^^^^^^

 

[GreggNY]

Congratulations, you just made the cheap bastards headspace gauge.

Thanks for the writeup, big. I'll be giving this a shot over the weekend.

 

[trauma1]

Tag


Trauma1

 

[MaxLoad]

I guess this is it in a nutshell. The fired cases are now exact to MY chamber, and just size back .001-.002 so they will chamber again. Correct?

What confused me was thinking that all factory ammo out of the box was sized to saami specs to chamber any rifle, and in bumping the shoulder, I didn't want to take the case all the way back to that short of a length. Since there are obviously variations in factory ammo cases as I found in my unfired 168 vs 175 FGMM, the starting point unfired length doesn't matter too much. Correct again?

Thanks again for the help guys

Pretty much correct on both counts... but fired cases are not exact to your chamber, but they are VERY close... It usually takes a few neck sized firings to really form that case to the chamber where you have a nice stiff bolt close. It really depends on how hot that load was, and the condition/brand of the brass.

I can measure unfired cases all day but again the don't provide me with any relevant information for my reloading or sizing. Just forget about unfired cases... for the purpose of reloading / sizing your brass to the chamber you are wasting your time.

Have you considered neck sizing your brass, until you actually have to FL size the cases? Depending on the application, it might be worth a look. For my dedicated target rounds, I don't FL size my cases until after the 3rd or 4th neck size.

 

[Killshot44]

And sometimes knowing how far the primer is protruding from the rear of the case can tell you far more than you thought it did.

As stated above only your fired and resized case lengths are all that matters, "BUT" knowing your chamber actual headspace length helps.

Resize a case to the shoulder bump setting you think is the proper shoulder bump length*. (minus .001 to .002 in a bolt action)
Yeah, no possible errors there....
Now measure the case and write this measurement down.

Now insert a fired spent primer in the primer pocket with just your fingers starting the primer into the primer pocket.

Now slowly chamber this test cartridge closing the bolt and letting the bolt face seat the primer and then remove it.

Now measure the case again from the primer to case mouth and write it down.

Then subtract the first case measurement from the second and this reading will be your head clearance measurement.

Sorry but there's NO way I'd use this measurement.

Why not use the tools that were designed to measure headspace to within .001"?
And if they don't understand headspace they have no business playing with gunpowder.

This is bad advice to give to new handloaders. Headspace issues lead to pressure issues which can lead to injury.

And unless they're removing the firing pin assembly from the bolt, this "measurement" is nonsense.

 

[bigedp51]

Sorry but there's NO way I'd use this measurement.

Why not use the tools that were designed to measure headspace to within .001"?
And if they don't understand headspace they have no business playing with gunpowder.

This is bad advice to give to new handloaders. Headspace issues lead to pressure issues which can lead to injury.

And unless they're removing the firing pin assembly from the bolt, this "measurement" is nonsense.

Killshot44, you are full of shit and do "NOT" know what you are talking about, and the firing pin has NOTHING to with using this method.

I have headspace gauges for the majority of all my rifles and I can tell you there are many ways to find out your headspace.

On a rimmed case all you do is measure your rim thickness and then add your head clearance and you have your exact headspace.

The only thing dangerous in a firearms forum are people like you who have no knowledge about the subject of headspace.

Headspace is also the distance between your ears and intelligence is the ability to use this space without sticking your foot in your mouth.

And unless they're removing the firing pin assembly from the bolt, this "measurement" is nonsense.

Gee I wonder why my firing pin is inside the bolt?