Head space comparator gage

[JScott]

I am getting ready for the upcoming Alabama turkey season.  This year I want to use my Thompson Contender pistol in .218 Bee.  One of the details for handloading ammunition for it is getting the correct head space so that the cartridges fire when they are supposed to and aren't over worked by being "stretched" during firing.  Keeping the head space at a minimum value makes this all easier.

To measure the head space I could have purchased a gage online but the design is very straightforward so I made my own.  The basic idea is a cylindrical gage that measures offset from a datum.  In this case the datum is the face of the barrel and the offset is the cartridge protrusion from the barrel.  The head space in my pistol is 0.010" which I measured with my feeler gage set.  I will load the ammunition so that the cartridges protrude 0.008-0.009" from the chamber.

Here is my version which should work for my simple application.  I used a piece of high strength bolt and did all the operations on the mill.  The set screw is 1/4-28 with a piece of lead shot at the bottom to protect the shank of the dial indicator.  The indicator is offset 0.139" from the center of the gage so that it rests on the head/rim of the cartridge case and not on the primer.  I will be using it this weekend so everybody keep your fingers crossed!

   

   

   

[rdhem2]

Hey Guy;
Neat idea.  Good way to go after your problem.  However from your pictures and description I don't follow your modus operando. With the gauge protruding so far through the base (?) what do you measure?  Maybe a picture while you are in action this weekend for us less imaginative.
Keep the SWARF flying!!!!

[JScott]

(04-05-2015, 10:18 PM)rdhem2 Wrote: With the gauge protruding so far through the base (?) what do you measure? 

rd, here a couple of pics/illustrations that show what the gage measures.

Head space is the distance from a datum in the chamber to the breech face.  In this instance it is the distance from the cartridge shoulder to the breech face of the Contender pistol.  If you have too much head space or "slop" the cartridges can stretch excessively or even worse they may fail to fire.

   

In this picture you can see the cartridge in the chamber of the barrel.  The comparator gage sits on top of the barrel and the stem of the indicator rests on the head of the cartridge.  By first resting the comparator gage on a flat surface and setting the bezel to zero it will tell you how much the cartridge case head is above or below the end of the barrel.


  Picture4.jpg (Size: 9.19 KB / Downloads: 139)

I wanted to set the head space to a maximum of 0.002" for my Contender.  I measured the barrel to frame gap with feeler gages and came up with a gap of 0.010".  I set my dies and reloaded the ammunition so that the cartridge case head protruded above the face of the barrel 0.008" to 0.009".  I have fired 3 of these rounds and they all look great so it appears that my gage is working!

I hope this explanation helps.

[Highpower]

(04-06-2015, 02:57 PM)JScott Wrote:

(04-05-2015, 10:18 PM)rdhem2 Wrote: With the gauge protruding so far through the base (?) what do you measure? 

rd, here a couple of pics/illustrations that show what the gage measures.

Head space is the distance from a datum in the chamber to the breech face.  In this instance it is the distance from the cartridge shoulder to the breech face of the Contender pistol.  If you have too much head space or "slop" the cartridges can stretch excessively or even worse they may fail to fire.

I'm afraid you've lost me.

The .218 bee is a rimmed cartridge. But you saying you are resizing your cases based on the cartridge 'shoulder' up by the neck of the case? Or by 'shoulder' do you mean the barrel counter bore? I'm not sure which one you are referring to.

If you have excessive head space, either the rim counter bore is too deep in the barrel or the barrel pivot pin / lug hole / frame holes are worn allowing the barrel to move forward. I don't understand how you could resize your cases (making them longer) without leaving a portion of the cartridge case ahead of the cartridge rim unsupported, and subject to separation.

Color me confused.

[rdhem2]

J. Scott, Sir:
I am afraid my band wagon follows High power.  A rimmed cartridge headspaces on the rim.  You must be short sizing them and allowing the small case shoulder to hold the cartridge where you want it.  That seems to me that this arrangement would be more conclusive to the dreaded misfire you speak of.  A hard primer could cause the cartridge to ride the firing pin forward mashing the case shoulder into the chamber neck and deforming the brass.  

I do not think the .218 Bee is a high enough pressure cartridge to worry about case separation unless the brass is really old and work hardened.  After all it was pretty popular in the Winchester M92 and I believe your Contender to be much stronger then a M92 in that respect.

Now if you wish to speak accuracy you may have a point to having the round tightly fit the chamber.  Being a single shot, cycling function is not part of the equation so a stiff closing is not relevant.

I guess what threw me was the long protrusion of the gauges actuator rod.  Now seeing what it does I probably would have made the holder longer to protect/hide the shaft.

Just my two cents worth.          

You would hate to see a full dollars worth!

[JScott]

You fellas are both right.  The .218 Bee is a rimmed case, low pressure round.  But even though it will head space on the rim it will still head space off the shoulder of the case when resized properly.  By keeping the head space to a minimum I am trying to wring the last ounce of accuracy and case life out of these cartridges.

I bought 100 of these cases new 3 or 4 years ago.  Now they are only available on the auction sites and going for crazy high prices.  If I am careful with them they should last me the rest of my life!  I'll keep you posted on accuracy results which hopefully will be in the next couple of weeks.

[JScott]

Just a couple more pictures that show the components I am using and zeroing the gage.

Here is the gage zeroed on a kitchen tile (surface plate is on the way from Amazon Prime!).

   

And here is an assembled round.

   

[Highpower]

(04-07-2015, 08:30 AM)JScott Wrote: But even though it will head space on the rim it will still head space off the shoulder of the case when resized properly.

OK I think I know what you getting at now. But saying that you are 'head spacing' off of the shoulder is where the confusion comes in. As rdhem2 mentioned rimmed cases head space off of the rim (only). You can have 'zero' head space on the rim, and still have the case shoulder meet the chamber, yes. But technically pushing the shoulder forward is not adjusting head space.

So the question becomes, what is the actual (rim) head space on the gun now? 
SAAMI specs .065" - .072"

It's different story for rimless or semi-rimmed cases that actually do head space on the case shoulder. I have a .223 barrel for my Contender. Do you really have .008 - .009" protruding from the barrel face?

You might try putting a ring of plasti-gauge around a case body ahead of the rim and chamber it, then close up the action. Remove the case and measure it to see how much clearance you have on the rim.

[JScott]

Willie, yes the head of the cartridge is protruding 0.008" above the face of the barrel.  And yes, technically the original use of the rim on this case is for maintaining proper head space.

My method of sizing the cases uses the shoulder instead of the rim to head space the cartridges at 0.001" to 0.002" from the breech face.  Now, the rim on my .218 Bee is used only for extraction and there is indeed a gap ahead of the rim.  rd's concern about the head of the cartridge having less than full support is correct but it is only missing full support by 0.008" and since this is the web where the cartridge case wall is strongest it won't cause any problems.

All this typing is making me thirsty! 

[rdhem2]

April 7th is National Beer Day you know.?  Prohibition ended 4/7/33 as if I need a reason to down some suds.      

I personally do not care for Contenders as the grip I use either opens the action about half the time or bangs my knuckles so I really do flinch. Badly.  Two friends of mine do well with them.  One in .30 Herret is hell on pistol silhouette targets and another turns in some very respectable 100 yard groups in .223 Rem.  I'll stick with my .38's, .44's, and .45's.  Ninety percent revolvers.  Yes, they do still exist!

Go Get 'em Tiger.  Keep us all posted on the results.