Semi-Automatic Mechanisms and Accuracy

[munir]

I rented an H&K P7M8 at the range tonight,
and it was a blast to fire. My son and
I really like it (the first SA auto for
both of us).

I noted that the H&K had a fixed barrel
(as does the Vektor CP1). Knowing nothing
about handguns, I would think a fixed barrel
to be inherently more accurate than a barrel
that tilts or rotates, or moved back and
forth. Just looking at the designs, I
would expect a hierarchy from more to less
accurate of:

fixed barrel
rotating barrel (very little lateral movement)
sliding barrel (lateral movement)
tilting barrel (lateral movement + tilt)

Is this train of thought incorrect? I
know that there are in fact representatives
of all of these available, from the
H&K P7 to the Beretta Cougar, to the
Beretta 92 FS to the 1911. I don't have
the sense of the fixed barrel dominationg
with regard to accuracy, which is what I
would expect. Do the fixed barrels dominate
with regard to accuracy, and I just don't
know that yet? Are there too many other
factors to permit a direct comparison?

Thanks.

 

[Mylhouse]

It would seem to me that if you had the FRAME of a fixed barrel gun mounted in a ransom rest, then you couldn't get any more accurate than that. But you aren't going to be shooting it in a mechanical rest. Remember, the sights are on the SLIDE, and unless the fit to the frame is perfect, you will get a slight deviation. As far as the tilting barrel (ala Browning), barrel to slide lockup is the key. The quickest accuracy fix for a tilting barrel handgun is to get an oversized aftermarket barrel that tightens up the slide/barrel fit. Some guns such as Glocks, can be fitted with an aftermarket barrel, and become true tack drivers, but mainly free hand and not in a Ransom. In a ransom, the glocks aren't so great because of the less-than-perfect slide to frame fit and the flexion of the frame during firing. I'm sure some will disagree, but that's my 2 pesos.

 

[Art Eatman]

And if you read about the various pistols which have been worked over for absolute best accuracy, the semi-autos often beat out revolvers.

Didn't BarSto at one time guarantee 2" at 50 yards with his barrels in 1911s?

But like Mylhouse said, a uniform tight lockup with respect to the sights is the most important factor. Everything returning to the same place, every time. Now, it may well be that the more monkey-motion, the more wear--but that's a whole 'nother story.

FWIW, Art

 

[James K]

Hi, guys,

If no one minds, how about a little discussion for the newbies.

When a gun fires, the powder in the cartridge case burns, creating high pressure gas that drives the bullet out the barrel. But the case also pushes back against the breech face and tries to back out of the chamber. If it is allowed to do so while pressure is high, the case will burst and release hot gas into the action.

In auto pistols, if the cartridge is low power (.22 LR, .32ACP) the gun designer uses a slide heavy enough that its own mass presents enough resistance that the case will be held in the chamber until the pressure drops. This is called a "blowback" system, since the case literally "blows" itself out of the chamber.

But if the cartridge is more powerful, a means must be used to lock the barrel and breech (slide) together until the pressure drops. The unlocking is done by the recoil caused by bullet movement (not chamber pressure), and this system is called a "recoil operated" system. The Model 1911 types and such guns as Lugers, most Berettas, etc. are of this type. It is the fact that the barrel and breech must remain locked for an interval that disallows a fixed barrel.

But there have been blowback pistols built for the 9mm Parabellum, and it is sort of on the edge between a cartridge that must use a locked breech and one that may not.

The H&K P7 deals with the problem in an unusual way. Its barrel is fixed, and the slide relatively light. But the barrel is rifled with very deep grooves which allow a good part of the gas to escape around the bullet, thus reducing chamber pressure to a point that the blowback action can handle it. But there is no such thing as a free lunch. The result is lower bullet velocity and power, and a high degree of wear on the barrel from the erosion ("gas cutting") caused by high pressure gas moving rapidly past the bullet.

Jim

 

[SAWBONES]

To Mr. Keenan;
With all respect, your description of the P7's mode of operation is almost entirely wrong.
It IS an unusual design. The barrel is pinned to the slide, and does not unlock, or move in any way, as you noted.
BUT, there is no rifling with "very deep grooves", and there is no "gas escape... around the bullet", there is no "loss of bullet velocity and power" (indeed, bullet velocity in the P7 series is higher than in other 4" barreled 9 mm pistols), and there is no "high degree of wear on the barrel". Your understanding of the mechanism is quite erroneous.
At the front of the chamber, immediately in front of the case mouth, at the lower part of the barrel, there is a small hole which communicates with a gas chamber beneath the barrel, running parallel to the barrel. This gas chamber receives a piston, which is attached at its front portion by a hinge to the slide. The barrel rifling is polygonal, without any traditional lands & grooves. When the cartridge fires, a small portion of the explosive gas "bleeds" off through the hole into the gas chamber, which produces a measure of gas pressure on the face of the piston contained therein. This holds the slide in place, and keeps the action locked, UNTIL pressure drops sufficiently to permit the force of the case head acting on the breechface to exceed the force retarding the slide. (At this point, the bullet has long left the barrel.) Then, the slide reciprocates, and the case is ejected. (The extractor is actually there for an extra margin of safety; it's not needed, and the gun functions perfectly without it.) As the bullet travels down the barrel, there is NO escape of gas around the bullet at all. On the contrary, the gas seal behind the bullet is superior to that achieved with traditional lands-and-grooves rifling, since the polygonal rifling permits a tighter fit of the bullet to the barrel, and therefore results in a higher bullet velocity. There is LESS barrel wear than with traditional rifling.
I have no interest in argument. I post the above simply to set the record straight about the method of operation for the P7. Its engineering design is superior, and accomplishes the desired result of superior accuracy & precision in a compact, lightweight 9 mm pistol. The design incorporates numerous additional interesting aspects, including the "squeeze-cocking" mechanism, that would take too long to go into here.


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"Potius sero quam nunquam."