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shot start presure, bullet jump, complete combustion
Discussion regarding the reloading of ammunition and tuning of loads for accuracy
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44marty
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PostPosted: Fri Sep 17, 2010 6:02 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Gain twist barrels seem gimmicky to me. Like Gelan says, this seems counterproductive. A slight gradual increase in land width combined with a matching (by volume) increase in rifling (groove) depth would seem better, to ensure perfectly forming the bullet to the barrel and maximizing spin. How in Hell would you machine that, though? Talk about an expensive barrel!

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Last edited by 44marty on Sat Sep 18, 2010 8:33 am; edited 1 time in total
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gelandangan
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PostPosted: Fri Sep 17, 2010 11:40 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

fnuser wrote:
have you ever recovered a projectile from a gain twist barrel?

No I have not. Nor have I know anyone with a gain twist barrel.
Have you? care to share pictures?

My posting above about Gain Twist Barrel is based on physic theory.
I would love to be proven wrong, for I am here to learn.

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fnuser
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PostPosted: Sat Sep 18, 2010 1:30 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

well I'm no psychic, and I am not trying to be contrary but since you raised the subject I was wondering if the engraving on the bullet was shaped like a wedge or a bow-tie, all depending on what the pivot point of the rifling i guess or another option would be that the obturation might kinda "heal" itself. Just curious as yourself I guess. I knew an old guy that shot 22-250 at p.d.'s that swore by it, his was built on a Remington 40 action complete w an old school Mcmillian stock. When he died I was able to purchase one of his Mannlicher Schoenauers from his widow but was out of cash after that and didn't want to try to bargain with a widow, I was thinking of my own wife after I make it. Anyway just like you said "theory" but would love to satisfy that with a real live observation. I'm guessing wedge since the rifling would start pretty close to the line of the bore and then exponentially increase in angle. The only people I know that still crank them out are Montana Riflesmiths. I wonder if the 2 pressure curves might equalize each other as the rifling might generate more as the projectile progressed and the peak from the powder ignition would be playing out. Anybody?

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slimjim
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PostPosted: Sat Sep 18, 2010 3:38 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

www.riflebarrels.com/a...curacy.htm

"However Lilja has quit using a gain twist. After years of comparing the accuracy of barrels with a gain twist and barrels with a standard twist, he has decided the gain twist offers no accuracy advantages. "You get just as good or better accuracy with a standard twist," he said, "just as long as the twist remains exactly the same the entire length of the bore."


These guys still make gain twist or "T" style rifling (transitional). They also say this type of rifling works best with drive bands.

www.bartleinbarrels.com/index.htm


It looks like it has been successfully used in recent competition for very long bullets that need extra stabilization. I assume from the discussions that the slower initial twist rate helps reduce overal pressure.

en.wikipedia.org/wiki/...pua_Magnum

"Rifles chambered for this wildcat cartridge, with a cartridge overall length of 105 mm (4.13 in), and equipped with custom made 180 mm (1:7 inch) progressive twist rate[30] 900 mm (35.43 in) long barrels with a 2° cone-angle (the standard C.I.P. cone-angle for the .338 Lapua Magnum is 6°) cone area finished first and second at several long range competitions. Its most recent win (2007) was in an international Special Forces and police sniper competition in Switzerland against rifles chambered for 7.62x51 mm up to .50 BMG at ranges from 100 m – 1,500 m (109 yd – 1,640 yd). The LM-105 bullet exhibited its very low wind drift susceptibility notably at ranges beyond 800 m (875 yd)."


I did not find pictures of bullets. Not sure you would notice the difference with a drive band.
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stovepipe
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PostPosted: Mon Sep 20, 2010 6:27 am    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

wncchester wrote:
Obviously, more consistant power burn will produce more consistancy of both velocity and accuracy potential.

Seating to the lands of your Mossy, or any other rifle, increases bullet start resistance, which increases start pressure and that may also increase burn consistancy, depending on several other variables including bullet weight and, to a small degree, a crimp. Increasing the intitial pressure can also be accomplished with faster powders and primer changes.

Any burn pressure that exceeds the yield strenght of the brass is going to fully seal the case to the chamber wall and that pressure is much lower than normal chamber pressures. Any slight smokey residue on a neck and shoulder is trivial and can occur after the bullet exits the muzzle.

Cool
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slimjim
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PostPosted: Mon Sep 20, 2010 6:19 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

wncchester wrote:
to a small degree, a crimp.


Last edited by slimjim on Wed Sep 29, 2010 7:15 pm; edited 1 time in total
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slimjim
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PostPosted: Mon Sep 27, 2010 8:14 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Time to ignite the discussion, no pun intended. I've been looking into .458 SOCOM or the .450 Bushmaster "Thumper" as a future AR project. While looking at .450 load data I found this on the net.

275gr Barnes 2.20 OAL

34.5grs Lil' Gun
2050 fps no secondary crimp
2093 fps light crimp
2128 fps heavy crimp

This reloader found higher velocities basd on crimp. No info on standard deviation or number of rounds in the measurement. The .450 case has no shoulder and is not that large a capacity. These rifles often have 16-inch barrels. The crimp in this situation may have more affect than in a case like my .270.

I still haven't found a target shooter that I've bumped into locally that crimps. I will keep searching. I don't have a crimp die but may start looking just to collect data for myself.
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44marty
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PostPosted: Tue Sep 28, 2010 4:58 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

That's pretty cool. Looks like I'm not the only one who has a crimp-crazy rifle. I wonder if the deviation got smaller with increased crimp as I found with my 300 WBY loads.

My K-31's and my .204 never get a crimp - it's just that WBY that "insists" on a crimp.

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slimjim
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PostPosted: Tue Sep 28, 2010 5:28 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Here is what a heavy crimp looks like on the .450 Thumper. A bit more robust than a crimp at the end of the case. For more detail see.

450bushmaster.net/view...p?f=5&t=24



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Ominivision1
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PostPosted: Thu Sep 30, 2010 8:09 am    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Very interesting subject,

I read the book Modern Hand-loading by George C. Nonte jr and he quotes as follows:

In modern practice, the bullet is permitted a modest amount of free travel within which it picks up velocity and momentum before engaging the rifling. This initial unrestricted movement allows powder gases to begin expanding early and thus eases somewhat the pressure peak. The amount of free travel is generally established by the rifle manufacturers based upon the longest and bluntest bullet which will be loaded in a given caliber. Consequently, the amount of free travel does not remain constant for other loads and other bullets. Shorter or more sharply pointed bullets will travel further before engaging the rifling. All other factors being equal, the greater the distance the bullet travels before engaging the rifling, the lower pressure will be. Many wildcatters and some foreign gun makers deliberately "freebore" barrels for a substantial distance for the sole purpose of holding peak pressures down and allowing maximum velocity to be obtained. In addition to the leade (throat) profile furnished by the manufacturer, one must consider that the throat is advanced by extreme shooting with high intensity loads. The beginning of the rifling is simply eroded away to the point where it substantially increases the distance the bullet must travel before engagement with the rifle. A given load will produce x-amount of pressure when fired in a new barrel, but after 1000 rounds or so have been fired in the same barrel, less pressure will be produced. It is practice for shooters to compensate for barrel erosion be seating the bullet farther out of the case as erosion progresses in order to maintain a standard amount of free travel. However, in order to retain the same performance, the powder charge must also be increased.

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slimjim
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PostPosted: Thu Sep 30, 2010 5:31 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Ominivision1 wrote:
All other factors being equal, the greater the distance the bullet travels before engaging the rifling, the lower pressure will be.


this is an important aspect of loading Barnes' all-copper bullet. They need to be further from the rifling than lead-core bullets to control pressure. Fortuneately I've found their "sweet" spot is also away from the lands.

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