Comped gun requires more power?

[Fred63]

Looking for a minor load for SC and NRA Action for a Czechmate, trying to weigh up benefits of 135g with faster powder v's 115 with WAC.

Can anyone explain why loads that will cycle a non-comped gun quite strongly struggles to cycle and has weak ejection in a comped gun?

Im assuming 5 inch barrel on similar guns.

[Hi-Power Jack]

2 hours ago, Fred63 said:

1,  benefits of 135g with faster powder v's 115 with WAC.

2.  why loads that for a non-comped gun  has weak ejection in a comped gun?

1.  The lighter bullet requires more grains of slower powders, producing more gas which "works the comp" better.

2.  Could be springs from gun to gun or reciprocating mass, but also you lose gas via the comp which weakens

      velocity and ejection - therefore you need more powder.

But, the difference wil probably be small from heavier bullets vs light bullets with the heavy gun and the comp,

and such light loads.   If you get the PF up a bit, the comp will work a little better, but my experience is that you

lose that with the heavier load - I don't see a significant difference between running very light PF loads and PF 150 PF

loads in my comp'd guns.

 

[Fred63]

Trying to understand the physics of it, a minor load with comp on barely cycles, some fte (not many, 1/100) and brass only flies 6" unscrew comp, screw on nut (Czechmate) same gun, same springs and same loads cycle well same as in my sp01. I still want to shoot with comp on as even with 3.6g ADI AP70 it has noticeably less muzzle flip.(maybe just the weight?) The nut does not change barrel length and once projectile leaves the barrel all gass is vented unhindered to atmosphere so I don't see how having a comp added on end can reduce pressure further or faster. Or is there a different reason I'm not aware of?

[GrumpyOne]

The comp delays the rearward motion of the slide some, keeping the gun "locked" for longer...not much, but some. This means there is less gas pushing backwards on the slide (that gas is forced through the vents on the comp). That's the same reason you need a lighter recoil spring for the same load in the same gun with a comp on it. Instead of those gasses (uncomped) forcing straight back, they are forced sideways and up (comped).

[SCTaylor]

Awesome explanation Grumpy! I've never really understood how the comp impacts cycling.

[superdude]

Just to add to the explanation, the gas force hits the baffle plate(s) in the comp and pushes forward on the comp. This forward force partially counteracts the rearward force created by the round's recoil force (a function of bullet weight and speed), and slows down the rearward speed of the barrel/slide.  

 

[zzt]

Fred, remember in Physics class you were taught that for every action there is an equal and opposite reaction.  In the case of your uncomp'd shot, this is fairly simple.  You pull the trigger and the gun goes bang.  The bullet is propelled forward by a force of x.  The case is propelled rearward by the same force, x, and it pushes against the breech face and eventually causes the slide to move rearward.  The bullet exits the barrel and the remaining gas in the barrel jets forward, while still pressing against the case.  You truly have equal and opposite reactions.

The slide has not begun to move when the bullet exits, or if it has because of geometry tweaks, it is a few thousandths of an inch.

Now take the case of the comp'd barrel.  Pull, bang, bullet moves down the barrel.  Everything is exactly the same as before right up until the bullet enters the first chamber of the comp.  As it transits the chamber, two things happen.  First, the gas is redirected up and out the comp port(s).  That reduces pressure in the barrel, so there is less of it to push against the case.  Second, the gasses press against the forward face of the baffle, and that effectively pulls the gun forward.  The bullet continues through the next comp chamber and more of the same happens.  In a properly balanced load for your comp, almost no gas exits the front of the comp.  Since almost all of the gas was directed up or sideways, and the barrel, including the slide, was being pulled forward by the actions of the comp, there is nowhere as much energy available to push the slide to the rear.  

The upward motion of the gasses redirected by the comp pushes the muzzle down.  The forward motion of the gasses hitting the comp baffles causes an equal and opposite reaction and pulls the barrel/slide forward.  So you have essentially taken a lot, or most, of the energy available to drive the slide back in the uncomp'd gun and redirected it up and forward.  To compensate for the reduced rearward energy available with a comp, you need to got to a lighter recoil spring.

[BeerBaron]

zzt nailed it. saved me the effort to post.

also, AP70 (equivalent of universal) is not the ideal powder for your compensated czechmate. I'd be looking at winchester auto comp, VV 3N37 or 3N38 or ADI AP100 if that is all you can find. 

[Fred63]

6.8 wac 124g gives 175pf for my open load but am also looking for a steel challenge load. My old loads for sp01 are very mild 132pf and dot hops up and back my only concerne is reliability with weak ejection. I'm trying to decide between lighter projectile same powder (6.8wac) or heavy projectiles and small load of fast powder that gives a very different recoil impulse. I know more gas will probably technically shoot flatter but in SC u transition to next target as soon as shot breaks so fast follow up shots don't apply. 

[BeerBaron]

sorry fred I didn't read completely. 

I tried this path in my old open gun and tried a load of 135n RN CMJ with universal/AP70. It wasn't bad, but was not all that accurate and whilst the gun was soft to shoot it had more muzzle flip than a major load.

The heavy bullet is a big factor for muzzle lift.

If loading for SC I would suggest stick to WAC but use the lightest bullet you can that is still accurate (i.e. still has a decent size bearing surface).

Start with a small batch of 90n bullets with about 6.5gn of WAC (or even try your regular 6.8gn so you don't have to mess with powder drop for the test). 

If they are no good try 100gn/110gn/115gn. You should be able to find pills in those weights either coated lead or JHP/CMJ (assuming no FMJ's for SC).

A 90gn pill going 1400FPS is crazy soft (may not even cycle the gun). almost no recoil and no muzzle lift. that's about 125pf.

The fast powder with heavy bullet feel soft in non-comped guns but you really notice the heavier bullet weight in comped guns.

slow powder with light bullet is the way to go. since there's no PF for SC you just go even lighter on the bullet weight. it will shoot like a laser.

 

[MrPostman]

115 grain and 8.5 grains of HS6 @ OAL of 1.165 should be wonderful out of your Czechmate.

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