Using a small base die isn't a bad idea for a semi. If your chamber is mil-spec or Wylde then you may get away with regular full length dies.

FWIW I use SB dies for my .38 bolt rifles.

FWIW, I have built and loaded for AR's for 20 years, I always use Small base, full length dies for ALL my AR loads for reliability. I also have been useing the Lee FCD with my varmint loads. I do not crimp my match loads, the match bullets dont take to kindly to being mashed, crunched, or smashed. So NO crimp of any kind on my MATCH LOADS!
Thats my 2 cents, you do what you want.

I load for a couple Ar-15s and a few other semi-autos as well. I have never found the need for a small base die. I do however crimp all my semi-auto loads with the Lee Factory Crimp Die. I have found the LFCD keeps the bullet secure and Increases accuracy.

Here is what the experts at Sierra have to say about it.

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.

Thanks Steve! Good info! Guess I better start looking at crimping my 5.56 loads...

Hmm!?! I have noticed that the cartridge lenght of my hand loads was longer than I set them when I extracted an unfired round out of the chamber. The bullet had marks on it like it had contacted with the lands and I assumed this contact/friction was what had cause the bullet to move forward out of the case. I subsequently increase my seating depth to avoid this. Looks like I may have to conduct a similar test to ensure what was causing my bullets to move out of the case.

It's when the first bullet stays sticking in the lands and you reload another round in the chamber, that the 'fun' begins: then you have powder everywhere inside + a new round with 2 bullets in the chamber...
nowadays a factory crimp die doesn't cost that much...
I've seen it happen in a BAR in .300 WM, loaded with the 125grs HP of Sierra. Lucky the guy didn't shoot that second round.

FWIW, I crimp every round I load. I have an AR, a mini-14, a couple bolt actions and a lever action. I use the crimp to snug the neck the same for each round. I figure it's 1 more thing I can do to make each one uniform. you don't need alot of pressure. You don't want it so tight that you cant pull the bullet if you have to. I doubt it makes a big difference in accuracy, it's just something I prefer to do.YMMV.
Best Regards,
CB

slimjim wrote:

Hmm!?! I have noticed that the cartridge lenght of my hand loads was longer than I set them when I extracted an unfired round out of the chamber. The bullet had marks on it like it had contacted with the lands and I assumed this contact/friction was what had cause the bullet to move forward out of the case. I subsequently increase my seating depth to avoid this. Looks like I may have to conduct a similar test to ensure what was causing my bullets to move out of the case.

Slim Was this a compressed load? Sometimes uncrimped loads with compressed powder will grow in length over time just sitting in the box. The bullet may now contact the lands, where the original loaded length did not. To check this, measure cartridge length immediately after reloading and then again after a few hours, and then after 24 hours.

44M, I'll check this next time I do some loads. Thanks!

Slim....can't remember if you crimp your rounds or not mate...but a LEE Factory Crimp Die will solve that problem for you...and probably help your accuracy as well.

Cheers, Vince

I don't crimp as I don't have a press. I get good accuracy from my .270 loads without it but I'm learning that reloading for a semi-auto is a different ball game than a bolt action. I've gone back to just loading my hunting loads for my AR and using factory loads for everything else. I'm saving my brass for the day I get a press and upgrade my loading situation.

I have Loaded almost 5000 rounds of .223 for my AR15 all with Lee Pacesetter Dies, never a jam or FTE. I have tried crimping some but there was no discernible difference in accuracy at 200 yrds. My standard Load hits a 25" Gong at 700yds every shot. my standard load is a 75gn OTM BT in front of 27gn of H414, OAL 2.250, and a light taper crimp to ensure complete powder ignition.
I trim my brass with the lee gauge and uniform the primer flash holes with a lyman uniformer.

Point, what barrel length and twist do you have? Not sure my 16" barrel 1:9 twist would do that well with 69gr SierraMKs.

It is a 20" Stainless Bbl 1-8 twist chambered in 5.56, your barrel should do just fine, my buddy can shoot my ammo through his Stryker pistol and hits the gong 3 out of 5 times

I've read that 75gr bullets are a flip of a coin in 1:9 twist. I tried some 75gr SMK in my gun and they didn't do well enough to shot them any more. Sometimes I think an 18" barrel would have been a good compromise.