Come ye Barons of Ballistics

Talk about your 450b reloading experience, ask questions, etc...

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Come ye Barons of Ballistics

Postby Specter23 » Sat Oct 25, 2014 9:00 pm

Seriously considering a 450B and would build the upper. Barrel being the most important part.

Reading on this site I see some conflicting thoughts (or just my lack or understanding).
It seems WC is able to really improve reliability and increase loads with a longer gas system. This would mean higher pressure (with hotter loads) that would help cycle the action, but also less time that the bullet is between the gas port and the muzzel and a port farther from the chamber, potential causing short stroking - over simplified I know

My thought is that if I could get a barrel made at 16" with a midlength gas system, or a 14.5" barrel with a carbine gas system, am I asking for trouble, or potentially preparing for better performance?

- I currently reload .223 rem, 308 win, 30-06, and 300blk - I look forward to optimizing the potential performance of this round and possible set up.

Thanks for any thoughts
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Re: Come ye Barons of Ballistics

Postby Hoot » Sun Oct 26, 2014 7:41 am

Having recently tested one brand of rifle length gas system barrel and being in the process of testing another brand, I just want to interject a caveat about high pressure capabilities and no, its not about safety. What I am discovering is that there may well be a reason for the low SAAMI spec beyond safety. It's about Case Head Growth. If all you use is low mileage or new brass, CHG is not a big issue. I'm up to around 2500 450b loads now and I have saved and tracked my brass cycles. They come from the factory with an average CH diameter ranging from .4990-.4998. Each time you fire them, that diameter increases. If you're using high pressure loads every time, in as little as 3 reloading cycles, the CHG can be up to .502. The spec on the chamber is .501 +.002/-0. New barrels done on CNC machines are coming out at precisely .501. When a clean, resized case CHG has risen to beyond .501, they resist going into the barrel before firing and they really resist coming out afterward. If your loads stay down in the SAAMI spec range, cumulative CHG occurs more slowly and using brass in excess of 5 firings is easily achieved. My original Bushmaster barrel was a little loosey-goosey on the chamber and was more forgiving of cases on the high end of their CHG life cycle. These new barrels not nearly so.

The original carbine gas system provides plenty of extraction force applied earlier and over a longer period of time to the fired case. A rifle length system provides that force later, as the pressure curve is falling and not long before the bullet clears the muzzle and the pressure stops. With older (fatter) brass, this all can add up to short stroking or the case actually being stuck in the chamber, needing a tap with the cleaning rod to get out. While you can open up the gas port diameter to provide a greater impulse amplitude, it's duration is still far shorter lived than with a carbine length or mid length system.

While the power of the recoil spring can force a case who's cumulative CHG has reached and gone beyond the chamber ID, to seat, the rifle length gas system may not be able to cycle the bolt effectively enough to extract it, eject it and strip another round for chambering, once it is fired. We have a lot of historical data for carbine length gas systems in this caliber. Data for rifle length systems is still emerging and no data on mid length systems. Having an old barrel re-drilled for a rifle length system will produce a less finicky end product than a new barrel starting off with a rifle length system as the used barrel has accumulated wear, including the chamber diameter. For a new barrel, I would encourage a mid length choice as a compromise and an opportunity for us here to get some empirical data to chew on. If all you intend to use is new factory ammo or reload brass for only one or two cycles, then the rifle length system will work for you just fine.

Hoot
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Re: Come ye Barons of Ballistics

Postby 3rdgeargrndrr » Sun Nov 02, 2014 8:46 pm

I would recommend NO CHANGES to the standard carbine system if you are planning on shooting factory or reloads duplicating factory fodder.
The carbine length system works fine and is much easier to troubleshoot. Changing the gas length introduces a ton of variables and requires that you would need to tune your rifle
sometimes fairly extensively to get it to function reliably. (Read 4 or 5 range trips)
I wouldnt think a 14.5" barrel with a pinned on brake would have any difficulty cycling factory loads.
I am considering a 11 or 12" barrel with a carbine system but have some other projects to get off the ground first.
I have a barrel currently sitting on my safe that needs an upper to attach to ;)
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