FPS loss in high ROF setup

[wilko129]

My mp5k build, with 12:1 gears and shs high torque, is shooting at 300 fps when using 7.4v lipos (rof below 30) but when i switch to 11.1s, which i usually run it on, the fps drops to just 260fps.

I have made the tappet return spring much smaller (about 4 coils), so it returns faster as this seemed like the obvious cause of the issue, but the fps loss persists even after making it really tight and I am not sure if I could make it any tighter without damaging the sping.

I am only using an M100 spring in the setup (no short stroking), so was wondering if I could be experiencing an issue because of that-do I need a bigger spring to avoid the piston not fully returning or would this be very obvious if it was happening? The rifle shoots at 40/41 rps when the motor is fully powered so you get an idea of the setup.

Any advice on this is appreciated from those who have made high speed SSG setups before as I have never gotten to the point of needing to short stroke before.

[sj_asc]

Re: FPS loss in high ROF setup

*What piston are you running?
*What brand of spring, or are the coils Linear or non Linear?
*Do you know the total piston assembly weight (+head, bearing etc)
*Are you running a Ported Cylinder, if so what type (1/2,3/4,4/5)?


Ported Cylinders can reduce piston return times, as can reducing assembly weight.

At 40 rps, even stock Marui v3 tappet springs are sufficient for feeding and sealing, although excessive mag feed tension and bb weight can slow this.

The issue is most likely that the main spring isn't sufficient enough to keep the piston assembly in time, M100 would in my experience almost certainly be too weak to keep in time at 40rps. If the piston is being interfered on completing its compression stroke, peak pressure will be lower.

If you can get the piston set put to have a look at the rack gear you should be able to identify any timing issues as there will be damage to the teeth.

From experience in my own build (which has been running since jun 2008 @42rps), you can get away with full stroke, however the spring rate required to keep that in time in most guns would be too hot for site limits over here. I run a Prometheus MS120 spring in mine, and the piston set is short stroked 2 teeth, that pulls in just shy of 360fps w/.2g, 4/5 + (247x6.08mm). You have advantage in that (providing you are running the stock inners of 110 or 141mm) they are harder to achieve fps from regardless of cylinder configuration or inner barrel bore diameter, so if you were to step up to M110 or M120 + Short stroking that you would almost certainly still be within site limits.

[wilko129]

Re: FPS loss in high ROF setup

I was hoping you might see this thread and reply

In answer to your questions:
-JBU polycarb with full metal teeth recomended by zero one http://www.zerooneairsoft.com/produc...oducts_id=6285
-The spring is an SHS M100 (linear)
-I cant remember at the moment what the exact weight is but I am using a G&G POM head with ball bearings removed so not super light cause of the full metal rack and no swiss cheesing
-I switched from the stock 1/2 ported cylinder to a TM 3/4 ported one to match my 247x6.03 barrel

I have an Element M115 I can throw in to see if that fixes the issue?

[sj_asc]

Re: FPS loss in high ROF setup

I would definitely opt for a higher spring rate, the M115 (assuming that it is true to it's rating) would be ideal, however you would want to short stroke to retain some of cycle rate losses and of course to stay within site limits. 2 teeth should be sufficient based on similar configurations that I have put together in the past.

If you have removed the bearing block from the piston assembly, be aware that it can affect spring performance since its a spacer which in addition to the ball bearing block on the spring guide (each ball bearing block being 5mm) makes up for spacing of the TM v2/3 spec solid piston head block (9mm + 1mm washer on the plastic spring guide). If the spacing is less then you could end up with slightly less muzzle energy/FPS than the spring is rated for, or in this case of concern, slower piston assembly return times.

AoE correction is typically around 4.2mm of spacing, and in addition to the typical 5mm ball bearing block on the spring guide and with a 1mm washer in the piston assembly can draw close to even spec with regards to intended paired parts spec, however you can experiment with further spacing to increase piston speeds (particularly with a short stroked system) on the existing spring.

As for reducing piston assembly weight, this is the most tried and trusted method which does not affect structural integrity.
(picture thanks to Mike at Hivemind)
Lonex Red as pictured below, but same applies to SHS - which they now also offer as a pre-cut option from factory with their 15T pistons.


I have found this method usually looses you around 3g of weight (at least for SHS and RA FMR pistons - which follow a similar body to the JBU)

[wilko129]

Re: FPS loss in high ROF setup

I have tried with the M115 spring and bizarrely I am getting the same fps as with the M100 spring-260fps with a large 11.1v and 300fps with a smaller amp 7.4v, both of which are very consistent respectively. I have another M115 which i can try to see if this one was a dud but not sure how likely it is to causing the issue.

I have removed the bearing block from the piston but have got one on the spring guide. I have also added 3mm 70D sorbo to the cylinder head so I guess that makes me 2mm short or do I need to account for the amount of give that the sorbo has when under spring pressure?

Thanks for the info on swiss cheesing. Do you know what size the six holes need to be or what size drill bit you use?

[sj_asc]

Re: FPS loss in high ROF setup

If that M115 spring is true to its rating vs the M100 spring in there previously then that definitely points to another issue, however, I would still recommend that you use the higher rate spring if you intend on running the 11.1v pack to avoid premature failure as a result of piston timing errors. M100 strength generally not fast enough for 40rps regardless of stroke, piston weight or Cylinder configuration.

As for the FPS issues, it could be a number of things.
I apologise if some of this may sound simple, but we all tend to over look things - I will probably miss something here

* Piston guide clearances, with the shell screwed shut and with cylinder/piston assembly removed, is the piston smooth on its guide rails?

* Does the FPS deviate when you have a full mag compared to an empty mag (or for high caps, fully wound compared to nearly empty)?

* Again, as you tried, is the tappet spring sufficient? Is there any interference? Not rubbing on the sector gear face? Or top half of the switch assembly?

* Does Tappet fin clear the sector gear axle correctly when the two halves of the shell are together?

* Hop bucking, have you tried a different bucking? Some buckings have longer sealing lip which can cause compatibility issues with certain brands.

* Air-nozzle type, which type do you have? It's very rare, but I have had some round exhaust types that were very poorly designed and on the intake stroke the bb can be drawn back towards the nozzle with enough force to cause it to stick - again unlikely particularly so with a ported head and cylinder, but still worth checking over. Also, is it the correct nozzle type? MP5K nozzle is very slightly longer than the ones use in V2, and doesn't have the same taper/chamber alignment profile.

* Is the FPS deviation between battery types the same with different hop settings?

* Is the hop chamber stable and correctly aligned with the nozzle? If this is slightly off, it can interfere with the nozzles path, causing slower return times and fps inconsistencies, as well as issues with feeding with higher tension mags/full.

* is the nozzle smooth to press in and return? Even when you mimic a slight upward bias which would be the feed tension?

* Is the inner nozzle of your cylinder head (if yours is 2-piece) properly tight/secured? This is quite common with the nylon heads, and also can cause feeding issues, sealing issues and horrific inconsistencies due to mag feed tension affecting nozzle alignment, nozzle return times and subsequently seal.

Do you have a base spring that you know the true out out with in a similar configuration?

[wilko129]

Re: FPS loss in high ROF setup

* Piston guide clearances, with the shell screwed shut and with cylinder/piston assembly removed, is the piston smooth on its guide rails?-seems very smooth, even loose to the extent that it will fall by itself when the GB front faces downwards.

* Does the FPS deviate when you have a full mag compared to an empty mag (or for high caps, fully wound compared to nearly empty)?-tried gravity feeding, full wind and low tension (high caps) and all gave the same fps.

* Again, as you tried, is the tappet spring sufficient? Is there any interference? Not rubbing on the sector gear face? Or top half of the switch assembly?-Seems to move freely and return quickly.

* Does Tappet fin clear the sector gear axle correctly when the two halves of the shell are together?-Do you mean is it far enough to the rear of the GB? If so then yes it seems to move freely and sit just behind the sector gear axle.

* Hop bucking, have you tried a different bucking?-its a TM and I have checked to make sure its not dried out and has worked okay before.

* Air-nozzle type, which type do you have?-I am using an Element O-ring air nozzle for mp5k series which has worked in this build okay before.

* Is the FPS deviation between battery types the same with different hop settings?-I am using an R-hop with no hop up adjustment but set to lift 0.25s perfectly at around 330/350fps.

* Is the hop chamber stable and correctly aligned with the nozzle? If this is slightly off, it can interfere with the nozzles path, causing slower return times and fps inconsistencies, as well as issues with feeding with higher tension mags/full.

* is the nozzle smooth to press in and return? Even when you mimic a slight upward bias which would be the feed tension?-It appears to be and an upward bias seems to have no effect.

* Is the inner nozzle of your cylinder head (if yours is 2-piece) properly tight/secured?-Mine is a one piece metal one and seems okay.

Do you have a base spring that you know the true out out with in a similar configuration?-I dont have any builds which have a similar barrel length really but I have another Element M115 and a spring which puts out 330fps in a 40cm barrel G36.

Okay so I just tried putting an extra 3mm of spacers onto the spring guide with the M115 spring and it gave a slightly higher fps for each battery (10fps tops) but still around 310fps for the lower voltage and 270 for the 11.1v.

[sj_asc]

Re: FPS loss in high ROF setup

Do you have a base spring that you know the true out out with in a similar configuration?-I dont have any builds which have a similar barrel length really but I have another Element M115 and a spring which puts out 330fps in a 40cm barrel G36.
.

That may well be it then. Typically most brands M110's come in close to 400fps, and 120's just over. If the Element spring is weaker than rated then it's likely too slow.

[wilko129]

Re: FPS loss in high ROF setup

Okay I will give the other M115 a punt and if not go for an M120 from another brand. Just to clarify though, the G36 does not have an Element M115 in; I just meant I have a spring in there with a known fps output (not sure if what I wrote was very clear).