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Old October 9, 2012, 05:18 PM   #1
RH
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Semi actions

I believe the semi actions on most pistols, pistol cal carbines (hipoint 995) and rimfire rifles are of the simpler recoil or blowback variety, while centerfire rifles use the more complex gas and piston systems. Why can't a centerfire cal rifle run on a simple recoil or blowback system? Aren't these systems more reliable, fewer moving parts, more forgiving of dirt, than gas tubes and piston tubes?
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Old October 9, 2012, 06:22 PM   #2
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Rifles are generally much more powerful with high pressures that require them to remain locked long enough for the bullet to exit the barrel and pressure to drop. A blowback design using springs and the mass of the breech would require truly massive parts, and very powerful springs. One solution is to use roller locked breech mechanism, like a CETME or G3. This is a retarded blowback system, which still requires stiff springs to help control the eventual release of the breech block. It's a funky way to get it done, but the G3 was widely adopted.
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Old October 11, 2012, 10:25 PM   #3
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Look at a hi-point 9, 40, or 45. Those massive ugly slides are what it takes to run those calibers in a blowback design. Now imagine in a 30-06 or more.
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Old October 11, 2012, 10:26 PM   #4
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Look at a hi-point 9, 40, or 45. Those massive ugly slides are what it takes to run those calibers in a blowback design. Now imagine in a 30-06 or more.

http://en.wikipedia.org/wiki/Hi-Point_Firearms
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Old October 16, 2012, 10:02 PM   #5
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John - that's exactly what led me down this line of question. I have a Hi Point 995 and a Marlin 60, and wonder why they can't build a centerfire rifle caliber rifle with one of those simple actions. I guess I'm not a semi action expert to understand how that simple blowback cant apply to larger caliber. I need to do more research on semi action workings.
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Old October 17, 2012, 03:19 AM   #6
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Quote:
John - that's exactly what led me down this line of question. I have a Hi Point 995 and a Marlin 60, and wonder why they can't build a centerfire rifle caliber rifle with one of those simple actions. I guess I'm not a semi action expert to understand how that simple blowback cant apply to larger caliber. I need to do more research on semi action workings.
It's simply a matter of pressure and how powerful the round is.

For example, when the Russians developed the Makarov pistol, their 9x18 round was considered the most powerful that can used in a straight blowback pistol and still have a carry-able weapon. Decades later, Hi-Point introduced straight blowback pistols in 9x19, .40S&W, and .45ACP, but in order to do this, they essentially had to drop the notion of it being a carry-able weapon, as a result Hi-Point pistols seem to look, feel, and weigh more like power drills than pistols. The mass of the slide necessary to contain the increased energy that comes with more powerful rounds resulted in a much bigger, heavier firearm. Hi-Point showed that it's very possible to make a straight blowback pistol in .45ACP, it just isn't a very practical gun.

The same sort of thing would apply if trying to do straight blowback rifle in .223/5.56, 7.62x39, or (heaven forbid) a .308. It is theoretically possible to build, but the mass of the bolt necessary, and related characteristics would render it a relatively impractical rifle. The increased simplicity would not be worth it given the gains in weight and dimensions.

I guess I should also note that from a practical stand-point, most small-caliber semi-autos like the Hi-Point 995 or Marlin Model 60, despite being simpler in design, are actually more complicated to field strip than AK's or even AR's anyways.
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Old October 17, 2012, 04:23 AM   #7
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Rifles and shotguns can be recoil operated and some are.

The HK G3 is but with a slightly different system, which is a delayed blow back system.
The Barret M107 and the Browning M2HB are also both recoil operated which a different system which is found in some pistols.

So yes it can be done, but so many gas operated rifles seem to operate just fine that there seems to be no major reason to change.
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Old October 17, 2012, 03:05 PM   #8
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;)

In most cases the weight of the bolt would be prohibitive. I remember the M3 Grease gun we used had a two pound bolt. The Barret uses the short recoil Browning system where the barrel and bolt move a short distance locked together till pressure drops. I would pick a delayed or turning bolt system or even the tilting bolt.



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Old October 17, 2012, 03:35 PM   #9
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The Machine Gun, Vol 4, by LTC George Chinn, available as a free download at MilSurps.com, will explain the advantages and disadvantages of all the known mechanisms.
http://www.milsurps.com/content.php?...eorge-M.-Chinn)

As for simplicity, blow back operation is extremely simple, but because the breech block weighs so much, the rate of fire so slow, simple blow back is only used in limited applications.

Delayed blowback is simpler than gas operated or recoil operated, uses a lighter breech block, and was used in a number of fielded systems, the Schwarzlose, the Breda M1930, and the Japanese Nambu are examples.





These early machine guns all had a problem common with all high power delayed blowback machine guns, breech friction. The designers had to add oilers, which was a logistical and operating mess:

These are oiler mechanisms from in Chin’s Book:



As Chin said:



Prior to WW2 the Germans copied chamber flutes from a captured Russian cannon and used flutes in their delayed blowback roller bolt. The flutes break the friction between the upper 2/3rd’s of the case and the chamber, the case head is still the gas seal. This design put all the oiler based designs on the trash heap about 70 years ago, so long ago that they are virtually forgotten and today ill informed people make outrageous claims about oiled cartridges ruining firearms.




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Old October 17, 2012, 03:52 PM   #10
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If you look at a Ruger 10/22 carbine, you'll see the simpest recoil operated system. As soon as the round is fired, the bolt starts to open. The weight of the bolt (which is big) and the stiffness of the spring make sure it opens slowly enough so the cartridge case doesn't explode because it leaves the chamber before the pressure starts to drop.

Next you might look at one of the Glocks. When you shoot it, the mass of the slide and the barrel along with the stiffness of the recoil spring determine how fast the slide and barrel start to move. But considering the energy and pressure behind the 9 mm round (compared to the .22 LR) you'd need a very heavy slide and barrel and a very stiff spring. not what you want in a pistol. The bolt is unlocked from the slide after everything slides back a short distance and the wedge under the barrel engages a wedge in the frame that makes the barrel tilt and disengage (unlock) from the slide. The barrel stops and the slide keeps going. That distance (which translates into a delay time) is what keeps them from needed the heavy barrel/slide and stiff spring.

Centerfire rifle cartridges are even more powerful (higher pressures) and need more of a delay in unlocking until they travel down the longer barrel. One way of achieving this is the AR-15 where the gas exits the barrel near the muzzle into a port that goes back to the bolt. The bolt therefore stays locked until bullet is pretty well out of the barrel and the pressure goes down.

Just imagine a Ruger 10/22 carbine in .308. The receiver would be enormous because of the mass of the bolt. The spring would be so stiff you'd have trouble working it by hand. You have to have some sort of delay built into the system, and that makes the design more complex.
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Old October 17, 2012, 04:14 PM   #11
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How about a gas retarded, blowback system like the HK P7? I would imagine the size of the gas piston, and tube to be prohibitively large to handle .308 type pressures.
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Old October 17, 2012, 07:07 PM   #12
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How hot that P7 system in .308 cal would get is surely an issue. Even 9mm has thermal management issues in that the heat from the tube transfers to the frame.
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Old October 17, 2012, 08:36 PM   #13
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;)

Just look at the Styer VG-1 8X33 and and 9X19 GB pistol gas trap retard system. They worked but didn't catch on.

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