Headspacing insanity- please suggest

[BrokenWheel]

I am reloading 7.62x39 for an Arsenal AK47. I know thats blasphemy - but hey I reload for all my firearm calibers.

In the past to determine my headspacing measurement (so I can set up my resizing die and later the Giraurd trimmer) i'll take fire-formed brass from my rifle and using the Hornady's headspace adapter for my caliper, I get a measurement. From this value I'll set my resizing die. For example, when measuring the HS of my fired 223 brass they all came out within .002".

However I'm getting wildly variable results differing up to .015" from the 7.62x39. These crazy values are coming from new Laupa brass that all had the HS (I measured) prior to me shooting them.
The brass is not really banged up (I try to avoid them hitting the ground and possibly deforming).

The chamber is clean and I was shooting a mild load. Im at a loss.

 

[warhwkbb]

Have you thought of purchasing a case gage?

http://www.midwayusa.com/product/333085/le-wilson-case-length-headspace-gage-762x39mm

 

[BrokenWheel]

I have that model, but the problem is that rim on the 7.62x39 greats chewed up by the extracting mechanism, so while that would not affect the function of the cartridge in the AK, the resized cartridge won't fully insert into the case gauge because it gets hung up on the "rim" of the brass.

 

[Slamfire]

The only way to check the headspace of a gas gun is with a chamber headspace gage. You cannot use fired brass as a means of determining chamber headspace. This is because gas guns unlock when there still is residual pressure in the barrel. This is intentional and was measured in laboratories. If everything is correct and there is not excessive friction between the case and chamber, the residual pressure helps to pop the case out of the chamber, aiding in extraction and ejection reliability. The technical term is residual blowback effect. Too high of pressures at unlock, or too high case to chamber friction will defeat this beneficial effect resulting in torn case rims or just jams.

How this effects your measurements is that the case is stretched somewhat during extraction. Unlock occurs when there is just enough pressure to keep things moving but not so much pressure to burst the brass.

You won’t see this in bolt guns because by the time your hand touches the bolt handle, pressures in the barrel are ambient atmospheric.

 

[243winxb]

I was shooting a mild load

Brass may have expanded out, larger in diameter. This pulls the datum back if pressure is low. Try hotter loadings?? Add/edit-

results differing up to .015"

Compared to the new brass, is the head to datum longer or shorter.

 

[BrokenWheel]

For example, the laupa brass HS measured 1.219" pre-fired.

24 grains of AA1680 and a 123 gr. FMJ projectile gave me an extreme of 1.227 on the high end, down to 1.213".

A couple of months ago all I had was AA 5744 powder so I worked up a powder puff load of 18.5 grains with a 123 projectile (avg 1732Fps), oddly the new laupa brass all came out 1.224". So maybe the violent extraction of a standard load deforms the brass? Or what slam fire said...

 

[jr_roosa]

You can use the cases themselves as a gauge. Screw out your sizer a bit and take the longest case and size it to .010 over what you think the headspace should be.

I don't know how the bolt works in your rifle but there should be a way to see when it's completely locked. Remove the ejector too.

Your long case shouldn't let the bolt close and lock.

Start sizing it down by 0.001" until it just closes. That's your headspace. Repeat a couple of times to be sure.

The Hornady gauge doesn't measure exactly the number you get with an actual chamber gauge, but if you care, you can zero it using a chamber gauge. The absolute headspace number doesn't matter, as long as you use the same gauge.

J.

 

[243winxb]

7.62x39

A couple of months ago all I had was AA 5744 powder so I worked up a powder puff load of 18.5 grains with a 123 projectile (avg 1732Fps), oddly the new laupa brass all came out 1.224".

The data (avg 1732Fps) matches the 150gr bullet.

http://www.accuratepowder.com/wp-content/uploads/2010/09/WP_LoadSpec_7-2-13.pdf

The 5744 powder has a faster burn rate. Pressure would increase faster than 1680. This is why 5744 expanded the brass to the chamber using the starting loads. It takes about 3 firings of neck sized brass to fully expand to fill the chamber. http://www.saami.org/PubResources/CC_Drawings/Rifle/7_62x39.pdf

7.62 X 39mm
Barrel: 20” ¦ Twist: 1-9.5” ¦ Primer: WIN WLR ¦ Bullet Diameter: 0.311”
Case: WIN ¦ Max Case Length: 1.528” ¦ Trim Length: 1.518”
Accurate 5744 CUP
123 HDY SP 20.3 1,962 22.5 2,180 44,040 2.187
125 SIERRA SPT PH 19.8 1,930 22.0 2,144 42,430 2.165
150 SIERRA SPT PH 18.5 1,732 20.5 1,924 44,090 2.190
Accurate 1680
123 HDY SP 25.5 2,201 28.3 2,445 43,120 2.187
125 SIERRA SPT PH 24.3 2,160 27.0 2,400 45,190 2.165
150 SIERRA SPT PH 20.3 1,752 22.5 1,947 42,280 2.190

Try hotter loadings A 150gr bullet with faster powder, would expand the brass better. If you were using the 150gr. and not the 123gr.

 

[steve4102]

As pointed out above. The fired case from a Gas gun is not a "replica" of the chamber as it continues to grow above chamber dimensions upon extraction. Different powders with different amounts of "port" pressure and gas can cause different fired case dimension.

One way to find and exact match for your chamber and headspace gauge is to shut off the gas system and fire several rounds.

Another is trial and error. Size and check, size and check until the cases chamber and extract with ease.

 

[243winxb]

The fired case from a Gas gun is not a "replica" of the chamber as it continues to grow above chamber dimensions upon extraction.

Not true with an M16A1 carbine.

 

[BrokenWheel]

243win- interesting. The speed I quoted (1732 fps) is from my chrono trials during autumn.

I thought the burn rate between 5744 and 1680 is practically the same.

 

[243winxb]

I thought the burn rate between 5744 and 1680 is practically the same.

On the burn rate chart, they are next to each other. But looking at the data above, the maximum charges are different by 5.8grs. The velocity difference is 265 fps. This tells me the 5744 is faster, reaching maximum pressure sooner. 44,040 cup to 43,120 cup.