Neck size only for semi-auto

[300 RUM]

If one is hand feeding a semi-auto would there be a need to FL the case?

 

[Hopkins]

Only if the gun won't achieve full battery.

 

[300 RUM]

Only if the gun won't achieve full battery.

So basically make sure the rifle cycles ok, and if so then neck sizing is fine.

 

[effendude]

I would suggest you always FL size any brass for a semi-auto. Why take the chance making ammo that may not be safe?
Scott

 

[Erik Cortina]

If you are after accuracy, F/L size. It's a common practice even on bolt action guns.

 

[NorCalMikie]

The first time the bolt won't close and the round fires "out of battery" will be the last time.
Neck size if you "know" the shoulder is pushed back far enough and the bolt closes.
Only neck size if you "really" know what's going on.
Or, full length resize and be on the safe side.

 

[spclark]

Re: Neck size only for semi-autos

Gas guns are engineered to be safe (and as accurate - hopefully - for their intended purpose as needed) with cases sized to easily fit their chambers when the bolt closes under spring pressure alone. That they also are fitted with free-floating firing pins makes them more likely to fire out of battery (as noted) when the bolt closes if there's anything slowing the bolt from locking up.

Never a good thing to have happen, without doubt not worth the consequences to you or your shooting partners if you feel the extra effort to FL size isn't really necessary.

Bolt guns are more forgiving in that respect but using your chamber and bolt handle as an impromptu sizing die is also something to be discouraged. Tools are best used for the purpose they've been designed for & without need of undue force to achieve the end desired.

 

[300 RUM]

Personally I have always F/L sized my cases for all actions, I believe it's better to start from a given dimension rather then possible variances from fire forming. I started doing so years ago after my first chamber job. I started paying very close attention to head space when the bolt would not close on my newly finished first cambering of a 7mm rem mag, I was trying to get it to run like a 7mm STW. Hot loads do funny things to brass So yes having to push a few shoulders back early in my days of the learning curve was a very good experience.

The reason I ask is today at the range there was a gentleman shooting a DPMS 308 along side my personal AR-10 (i'm a big Armalite fan, can you tell) and I don't mean to offend anyone but shooting Win white box ammo is just not me, and he got just as big of a kick out of my 308's with Burger 185s hanging way the out ha ha I'm still laughing about that. Anyway he asked me if I was fire forming for the chamber seeing how I was hand feeding them. He explained that he was not a precision shooting but more for hunting and believe me the target showed it, I could of used his center. One thing that came to mind is maybe he knows something I don't.

 

[bowslngr]

I agree that there is nothing significant to gain from neck sizing for individually loaded rounds in a gas gun, but an out of battery fire is just not going to happen. The AR bolt & carrier design prevent this.

Here one way to consider the question. I'll bet every round you have ever extracted from you AR chamber without firing it has a very slight primer indention in it. That is caused by the floating firing pin as the bolt carrier comes to an aburpt halt on lock up. However, you never hear of ARs spontaneously firing upon lock-up. How is the fact that the bolt may not fully close going to make that change? The firing pin cannot achieve any protrusion until the bolt begins to lock and cannot achieve its full protrusion until it is fully locked.

 

[markm87]

The first time the bolt won't close and the round fires "out of battery" will be the last time.

Depends on the gun. An AR type rifle can not do this. The firing pin can't physically touch the primer until the lugs are in place.

But still... I'd full length size for sure. You'll end up getting a round you need to beat into battery, or rip the rim off trying to remove from the chamber.

 

[chuckw2]

I always FL resize for my auto' s and hunting guns.... I had a hunt almost head south on me in TX with ammo I could not get in my bolt gun without using a 2 lb hammer.
Chuck

 

[spclark]

An AR type rifle can not do this. The firing pin can't physically touch the primer until the lugs are in place.

I'd thought to disagree based on my own limited experience, but after putting the question to a highly respected AR gunsmith I understand now that this is in fact the truth. I stand corrected.

 

[300 RUM]

On a Ar-15 (small frame) semi auto you do see a lot of primer marks on the primer when you extract a chambered round you wish not to fire, and yes the odds of the hammer falling are slim to none by design, the BCG is in the way. The floating pin can cause problems and yes if the extractor catches the round and the BCG slams to a stop you can get a AD with the floating pin.

On an Ar-10 (Armalite) the pin has a retaining spring, it is not a floating pin, this does not mean it is fool proof. I have extracted a few chambered rounds out of AR-10s and I was surprised on how well the retaining spring works.

I only build on actual AR-10 uppers and lowers using only Armalite BCGs and bolts, I have no experience with other Ar type 308s

In the short anything can happen if "Murphy" allows it to. I will always F/L size just for the fact of starting with some given dimensions just like I will always tumble my cases, I like shiny things

 

[Laurie]

Here one way to consider the question. I'll bet every round you have ever extracted from you AR chamber without firing it has a very slight primer indention in it. That is caused by the floating firing pin as the bolt carrier comes to an aburpt halt on lock up. However, you never hear of ARs spontaneously firing upon lock-up. [Bowslngr]

Yes.

The British Army bought a few hundred early model M16s for evaluation some decades back and put them into service with an SAS (special forces) unit I believe somewhere where there was a low level shooting war going on and there was continuous patrolling with a round in the chamber.

Weapons would be made live at the start of a patrol by all of its participants and ejected on safe return at the end with no shots taken. The ejected round was invariably returned to the magazine and when the rifle was next used was of course chambered again. The light primer strikes built up until instances of spontaneous uninitiated firing on chambering such rounds occurred - a serious issue in any responsible military outfit and presumably with the unfortunate soldier(s) feeling the heat under their collars initially as being suspected of initiating a 'negligent discharge'. The cause(s) couldn't initially be determined until somebody noticed the light primer indentations and replicated the repeated chambering and extraction routine. I'm not sure what was done about this. Primers with harder cups?

I'm sure US forces saw similar occurrences in their much more widespread 1960s deployment of the XM16 in Vietnam.