Is there a value to throat diameter measurement?

[adam_mac84]

I worked out a usable seating depth with my 125 (dropping 130) GR .356 cast bullets.

I can coat and load as cast with OAL 1.050 and get reliable feeding. Over 1.105 I run the risk of failure to go into battery.

I cast a soft lead bullet and plunked. I did force into place lightly and scored the bullet. It is hitting on the shoulder that the case should be headspacing on. I did a few times to wear in a groove so to speak. Measures 0.3565 at the score marks. Is this the diameter of my throat then?

I had a difficult time slugging my barrel as it is S&W with 5 grooves. Can I leverage this measurement as a slug of sorts?

I loaded and shot 10 rounds today at the 1.050 length with 3.7gr HP38. Accuracy was acceptable. I am most concerned with a functional load at this point and trying to determine what sizer if any I should get. I am not at a compressed load. I have to give there
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[Outpost75]

Throat diameter is a more useful measurement. Measure your marked, chambered and extracted round and get a sizer which is about a half-thousandth to one thousandth smaller than what you measure and you will be fine.

[adam_mac84]

Looks like .356 it is then


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[243winxb]

Or seat the bullet deeper, shorter OAL. 1.050"

You want to find groove diameter.

[243winxb]

I had a difficult time slugging my barrel as it is S&W with 5 grooves. I cut a strip from a soda can. Wrap tightly around the bullet. Measure with a micrometer. Subtract the thickness of the strip X 2*. This should get you close to groove diameter. Bullets should be about .001" larger after sizing.

Ty mdi*

[mdi]

Hint; 243winxb's method works but subtract two times the thickness of the strip, one thickness on each side of the measured diameter...

[fourarmed]

I'd just seat deeper until it plunks. I think that extra diameter might help accuracy.

[adam_mac84]

I have gotten them to plunk by seating deeper. This was intentionally long to see what happened. I knew that over 1.105 would not go into battery. So was curious as to why, which prompted this test. Now I am trying to wrap my brain around why as the bullet comes out of the case it isn't just getting shaved off like this.

I did do some calculations that suggested that I shouldn't have major problems starting near 1.050 with low loads. None of my rounds showed any signs of pressure yesterday. Today I am going to seat a few at 1.050 at 3.7, 3.9 and 4.0g HP38 and test for pressure/leading

[mozeppa]

dougguy throats auto's

[wv109323]

You need to seat where the load passes the plunk test. Press the round into the chamber and make sure it falls out when vertical. If the bullet contacts the throat,lead will accumulate at the front of the chamber. After about 20 rounds you will get one that will not go into battery.

[popper]

The case 'should' HS on the chamber end. Many don't really have a throat, directly to bore so you get to size just above bore. If the bore has just a small bevel it might not shave the lead.

[DougGuy]

You really should have the barrel throated. Going smaller is just going to lead the barrel at some point you run out of anything gained by doing this. Seating deeper is a POOR workaround for a throat that is too tight for cast boolits, and you can very easily run into an unfortunate situation where you don't compensate enough with your load data and now you got an over pressure event. In so many words, the gun blows up in your hands. 9mm is a high pressure cartridge, operating at 35,000psi. Any boolit setback be it the feed ramp causing it, the throat jamming the boolit deeper into the case, or the operator at the loading press seating the boolit deeper, can cause pressures to skyrocket.

If you have a fitment problem with ammo assembled within published load data and SAAMI specs, then the problem ISN'T the ammo!

The proper thing to do is to have the barrel throated to accept the cast boolit you intend to shoot AT the diameter and COA you want to load it to. Throating the barrel reduces pressure, allows you to seat and chamber a boolit .001" to .002" over groove diameter of the barrel which allows the boolit to seal better than a smaller boolit, Longer COA usually feed much better, leading is a thing of the past and they shoot lights out afterwards. PM for details.

Springfield RO barrel, 45 ACP, 9mm is the same principle, stock factory as-issued barrel on the left, same barrel after throating on the right. Notice the rifling runs right down to the chamber mouth, there is zero freebore, this isn't even within SAAMI specs.

http://i1202.photobucket.com/albums/bb374/DougGuy/Cylinder%20Services/1f06c85f-3c7d-434b-9061-ca061af26de9_zps0aqi32iq.jpg (http://s1202.photobucket.com/user/DougGuy/media/Cylinder%20Services/1f06c85f-3c7d-434b-9061-ca061af26de9_zps0aqi32iq.jpg.html)

SAAMI specs call for a section of tapered freebore at the end of the chamber, to allow the boolit to be seated to the proper COA and still "plunk" in the chamber easily. If your barrel cannot do this, then fix the problem barrel, instead of modifying tested, proven, published load data to compensate for the barrel's lack of a throat.

[collyer]

Doug did 3 barrels for me, Excellent work and solved my problems. He has a quick turn around time.
Tom

[adam_mac84]

Thanks Doug. I will check mine. I will be getting a sizing dir this week. Will check again. I did run some tests with OAL at 1.080 today which did seem short, but based on my measurements shouldn't cause a compressed load. I worked up 3.7, 3.9 and 4.0gr hp38. All shot well enough but the 4.0 had significantly more smoke. Like noticeably more smoke. If I cannot plunk with a sized PC bullet. I will be getting in touch. For both my smiths. This whole cost savings thing is adding up. Gotta look at the long game here. (I know. It's not about the money. I do enjoy the process)


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