Speer TNT OAL growth when chambering

Moderators:gds, bakerjw, renegade, bamachem

Post Reply
Odysseus
New Member
Posts:4
Joined:Sat Jun 06, 2020 10:02 am
Speer TNT OAL growth when chambering

Post by Odysseus » Wed Oct 13, 2021 5:41 pm

Just getting started to load the TNTs, and cranked out a few dummy rounds to function check in the intended hosts (two different AR-15s) before live testing. Every time I dropped the BCG to chamber a round from the magazine, the OAL grew .004"!

The brass is virgin Starline.
Brass prep as follows: size in Lee FL sizer, chamfer with K&M tool, tumble in media 2 hours to remove sizing lube, load to OAL of 2.100".

Measured neck tension was .0025". I polished the expander down .0005" at a time, loading and testing rounds each time. I'm up to .004" neck tension and OAL still grows by .003".

I thought perhaps there just wasn't enough bullet in the case, so I adjusted seating depth for an OAL of 2.075". Chambered and got OAL growth of .015". What is happening?!

BulletFlight for Android
User avatar
BobinNC
Silent But Deadly
Posts:187
Joined:Thu Aug 16, 2012 2:35 am
Location:Goldsboro, NC
Contact:

Re: Speer TNT OAL growth when chambering

Post by BobinNC » Wed Oct 13, 2021 6:25 pm

Try a Lee Factory Crimp Die and a medium crimp. The Lee FCD does not require a cannelure on the bullet.

User avatar
rebel
Silent But Deadly
Posts:7274
Joined:Mon Apr 22, 2013 2:01 pm
Location:Moonshine Country

Re: Speer TNT OAL growth when chambering

Post by rebel » Wed Oct 13, 2021 9:54 pm

Maybe start with a light crimp with the TNT. Jackets are fragile on varmint bullets and easily damaged by crimping.
You can't beat the mountain, pilgrim. Mountains got its own way.

TRshootem
Silent But Deadly
Posts:247
Joined:Fri Dec 26, 2014 1:13 am
Location:Montana

Re: Speer TNT OAL growth when chambering

Post by TRshootem » Thu Oct 14, 2021 9:22 pm

Yes sir, the Lee factory crimp die is just the ticket for what you describe. It works very well, I load and shoot a lot of the Speer TNT's with no issues.

Odysseus
New Member
Posts:4
Joined:Sat Jun 06, 2020 10:02 am

Re: Speer TNT OAL growth when chambering

Post by Odysseus » Mon Oct 18, 2021 10:27 pm

So one problem was a stiffer than necessary buffer spring. I went to a standard spring and now OAL growth is down to .002" (from an originial 2.100" OAL).

After a little more research, I came across the following article from Sierra Exterior Ballistics:
Reloading for Semi-Autos and Service Rifles

Neck Tension

When we stop to consider the vigorous (read, downright violent) chambering cycle a loaded round endures in a Service Rifle, it becomes pretty clear it suffers abuse that would never happen in a bolt-action. This is simply the nature of the beast. It needs to be dealt with since there is no way around it.

There are two distinctly different forces that need to be considered: those that force the bullet deeper into the case, and those that pull it out of the case. When the round is stripped from the magazine and launched up the feed ramp, any resistance encountered by the bullet risks having it set back deeper into the case. Due to the abrupt stop the cartridge makes when the shoulder slams to a halt against the chamber, inertia dictates that the bullet will continue to move forward. This is exactly the same principle a kinetic bullet puller operates on, and it works within a chamber as well. Some years ago, we decided to examine this phenomenon more closely. During tests here at Sierra’s range, we chambered a variety of factory Match ammunition in an AR-15 rifle. This ammunition was from one of the most popular brands in use today, loaded with Sierra’s 69 grain MatchKing bullet. To conduct the test, we chambered individual rounds by inserting them into the magazines and manually releasing the bolt. We then repeated the tests by loading two rounds into the magazine, chambering and firing the first, and then extracting and measuring the second round. This eliminated any potential variation caused by the difference between a bolt that had been released from an open position (first round in the magazine) and those subsequent rounds that were chambered by the normal semi-automatic operation of the rifle. Measuring the rounds before chambering and then re-measuring after they were carefully extracted resulted in an average increase of three thousandths (0.003") of forward bullet movement. Some individual rounds showed up to seven thousandths (0.007") movement. Please bear in mind that these results were with factory ammunition, normally having a higher bullet pull than handloaded ammunition.

To counteract this tendency, the semi-auto shooter is left with basically two options: applying a crimp or increasing neck tension. The first option, crimping, brings up some other issues that can be troublesome. In general, crimping degrades accuracy. Most match bullets are not cannelured (which also seriously damages accuracy potential), a requirement for correct application of most crimps. Still, there are taper crimp dies available from most of the major manufacturers. Lee offers their “Factory Crimp” die as an alternative, which seems to be one of the better options for those bullets without a cannelure. That having been said, crimping is still, at best, an occasionally necessary evil. Avoid it if at all possible.

http://www.exteriorballistics.com/reloa ... reload.cfm


It seems that now I'm down to .002" (which is better than Sierra's results with commercial match ammo), I think I can live with that.

Post Reply