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 Post subject: .17 conversion
PostPosted: Fri Dec 25, 2015 1:40 am 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
If the symptoms of the 'mod flu' are, the very moment you finish one mod your looking for another I've caught it.
So far I've done grip mods, trigger mods, a nitro piston conversion and the '5 screw mod'. I'm new to the hobby but have been learning like a man drinking from a fire hydrant. I might not have the equipment or the skills or even the knowledge I need yet but there is no stopping me now.
I've been reading with great interest about the recent popularity of the new .17 cal. rimfire rifles. These ballistic tipped bullets rely on blistering velocity (2200 to 2600fps) to explode on contact.
Many of the rifles like the Ruger and the CZ offer guns that easily swap from .22lr, .22wmr or .17 hmr so the barrels are readably available. The bullets come in 17 and 20gr weights.
The .17 hm2 or mach2 are .22lr shells necked down to accept the .17 bullet. These cartridges generally produce about 100fpe. I've been thinking of converting a pcp rifle to shoot .17 bullets. My question is can air rifles today produce that much energy and velocity?

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 Post subject: Re: .17 conversion
PostPosted: Fri Dec 25, 2015 7:54 am 
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Joined: Tue Jan 29, 2013 6:54 pm
Posts: 2411
Location: Northeastern Ontario
The CZ 455 can swap barrels between .22LR, .22 WMR, and .17HMR. I don't think Rugers do that. In any case, PCP's can be had that deliver 100 fpe. The problem is that it is easier to achieve that level of power with larger calibers -- i.e. larger than .22, at the very least. The much smaller diameter .17 caliber is just not large enough to make a PCP delivering 100 fpe practical. Others will, no doubt, add further information and explanation.


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 Post subject: Re: .17 conversion
PostPosted: Fri Dec 25, 2015 11:59 am 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
I think I understand. Gun powder creates huge amounts of pressure, 24,000 psi for a .22lr cartridge, but a small amount of gas where as a pcp only makes at best 4,500 psi but has an abundant amount of air available. So a larger caliber will allow more air to act on the pellet for a longer time. That would also explain why a longer barrel in a pcp will shoot a pellet at a higher velocity.
Perhaps I should be looking at a multi pump pneumatic that can create much higher pressure but less volume like gunpowder does.
Can air be compressed to 24,000psi even if I can find a way to make it practical?

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 Post subject: Re: .17 conversion
PostPosted: Fri Dec 25, 2015 12:06 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8978
Location: Coalmont BC
Potential power from any PCP is proportional to caliber squared (bore area), barrel length, and pressure.... So, if you take a .25 cal shooting 100 FPE and put a .17 cal barrel on it, the most you are going to get is 46 FPE, with the same barrel length and pressure.... In addition, compared to PBs, we are working with tiny amounts of pressure, and that affects the Sectional Density of bullet that you can push at a given velocity.... We don't have the pressures available to reach 2000+ fps, and are further hampered by the speed of sound in air INSIDE the barrel, which severely hurts efficiency when you try and push past Mach 1.... Using Helium can avoid that, but to have any chance of doing what you want, you need to start thinking of a gun with a 36" barrel running 4500psi of Helium or more....

To answer your question (partially) about hyper-pressures.... Air starts acting more dense the more you compress it.... Even at 4500 psi, compared to 3000, you start noticing a drop in power from the same volume of air compressed to the higher pressure than theory (Boyle's Law) says.... The higher the pressure, the more noticeable the effect.... The highest pressures I have heard of for compressed air is about 10,0000 psi....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: .17 conversion
PostPosted: Fri Dec 25, 2015 12:41 pm 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
Wow. I see I have to drink a lot more from that fire hydrant. What I did understand is with airguns we need to stay subsonic. So to create higher energy we need to use larger calibers and heavier pellets/bullets. In the end impact energy is impact energy regardless of caliber so the effect of a .17 hmr (245 muzzle fpe) may be possible with a larger caliber airgun.

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 Post subject: Re: .17 conversion
PostPosted: Fri Dec 25, 2015 1:48 pm 
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Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
It takes a lot or pressure to force a jacketed bullet into rifling, and a lot to keep it moving. The smaller the bore the worse that gets. I seriously doubt a pcp filled to 10kpsi would be enough. So I'd bet $ that the bullet would partially wedge into the rifling and stop, then you'd have to pound it back out.


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