Perfect Neck Tension for Bolt vs. AR

[Big338]

Ok, so here is my quick theory on the perfect neck tension for the best accuracy out of a bolt gun. This is just my conclusion that I have come to over a few years of many rounds down range. I want to have minimum neck tension which means to me .001 neck tension if possible.

Here are a couple examples of developing my rounds with that goal of .001 neck tension in mind.

308 using Lapua brass which has a neck wall thickness of .014 X 2 for each side = .028. Add .308 bullet diameter to the .028 which means that 0 neck tension would be a .336 outside loaded neck diameter. I'm using a .335 neck bushing in a Reading neck only die which means I'm resizing .001 more than the .336. This is how I claim a .001 neck tension. This is what is shooting .5 moa most of the time out of a shilen 10 twist barrel.

The same math works for my 260AI. Same neck thickness of .014 with Lapua brass even though it's been necked up from .243, reamed, and turned. Anyway ((.014 X 2) = .028) + .264 =.292. I’m using a .291 Reading neck bushing die which again is a .001 neck tension. This is currently producing under .5 moa 5 shot groups all day long out of a Krieger 8 twist 30 inch barrel.

I ordered .289 and .288 neck bushings for my 6.5 Grendel AR which showed up today. The idea is that I want more realistic neck tension for a gas fed semi-auto AR platform. I should be able to get .003 and .004 neck tension for these rounds using the same math above. My question would be what neck tension is necessary to be safe in an AR platform with rounds that are not crimped?

 

[243winxb]

My question would be what neck tension is necessary to be safe in an AR platform with rounds that are not crimped?

Bullet pull of 35 to 45 lbs is needed in the 5.56mm. If the bullet expands the case neck .002" or more on seating, you should be in the range of 35 to 45 lbs. http://www.exteriorballistics.com/reloadbasics/gasgunreload.cfm

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.

The other—and in our opinion, better—option is increased neck tension. This, in turn, leaves us with two more options depending on what type of equipment you’re using. The object of either is simply a tighter grip on the bullet. Using conventional sizing dies, (i.e., those utilizing an expander ball) this is accomplished by reducing the diameter of the ball itself. This can be done by chucking the expander/decapping rod into a drill and turning it down slightly with fine emery cloth or a stone. The goal here is to decrease the diameter two or three thousandths (0.002" to 0.003") under bullet diameter. This is a trial and error process, and must be done slowly. The end result is an expander ball that opens the case neck up somewhat less than the as-issued item. This, in turn, increases the grip of the case neck on the seated bullet.

A better alternative to achieve the same effect is the use of a bushing die, such as those from Redding Reloading. This is by far the best solution, not just for Service Rifles, but for a broad range of reloading applications. The basis for this system is a fairly conventional sizing die, at least where the body and shoulder of the case is concerned. In the neck area, however, the die is fitted with a removable bushing. Available in .001" increments (as measured at the inside diameter of the bushing), they can be matched with a specific batch of brass to provide optimum neck tension. This tension can be increased or decreased by simply moving up or down in bushing size. The one drawback to this system, if it can be called a drawback, is the absolute necessity of sorting cases and loading them in batches. This, of course, is how virtually all loading should be done anyway.

 

[NCsmitty]

I try to maintain a .002" difference between expander ball and bullet diameter, and that makes it necessary to keep brass separate because of variations between manufacturer's of brass.
After decades of loading ammo, I maintain specific brands of brass for each of the calibers that I load. I never use mixed brass in my rifle loads, but that's just me.



NCsmitty