Sr-25 dmr

[henry1234]

Hey guys I would appreciate your views on this. I am planning on building an A&K SR-25 DMR, using the standard gun as a shell I would then build up the internals from scratch using the below components. My aim is to get it firing a consistent 500fps with a 0.20g bb so when using .30g it would give a good mix of range and stability. It would be gearbox locked to semi automatic so no possibility of full auto (which would be illegal) and a hard wired MOSFET limiting it to 1 shot per 4 seconds. I would also use all of the tweaks in the holy grail of DMR building.

The internals would be
- 650mm 6.03 Prometheus barrel with R-hop
- ICS M4 Hop up set
- G&P M180 SATAN AEG (Motor) Long
- Matrix SR-25 aluminium steel strip piston and piston head set
- Sorbo pad on piston head
- echo 1 tappet plate for sr-25
- SHS stainless steel cylinder for sr-25
- Element aluminium ultra v2 cylinder head
- element aluminium nozzle
- M150 spring
- 8mm metal bushings
- element shim set
- SHS X-Mod SR-25 steel gear set
- 11.1 V lipo
- silver low resistance wiring


Any other suggestions?
Any help is most appreciated.

Once the insides are done ill then go through the externals if you guys want?

[sj_asc]

Re: Sr-25 dmr

The SR25 box is a breat base to start with for a DMR since it has a larger capacity cylinder (PSG-1 type) which allows you to take advantage of better muzzle energy efficiency through the use of heavier BB's.
I would suggest that you use the shorter barrel, the longer inner barrel would provide you with higher FPS figures with the typical .20g, but not so much with heavier weights that you would likely be using. Marui spec'd that cylinder in the PSG-1 for a 590mm x 6.08mm inner barrel and for .25g bbs. You really would be maxing that out if you were set on using heavier bbs.

Consider these points;
*Heavier BB's require additional volume to retain efficiency
*Longer inner barrels also naturally require additional volume on top of this
*Despite the R-hop having a more efficient hop application, that hop setting still has to be more aggressive with heavier bbs which again requires additional volume to reduce the natural decline in muzzle energy that such setting brings - (which is potential range).

Ideally, if you were intending on using .3g+ I would strongly advise that you don't exceed 590mm. Stick with the Prometheus 6.03mm though , fantastic barrels!

Motor - G&P's M180 Satan is not worth the money that it cost's at all, thats over £100 for a motor that is poorly refined! They are also tuned more for speed than torque. If you are set on G&P, the M170 would be a better option for torque. TBH, I would suggest otherwise due to cost.
Lonex A2's are fairly well balanced and fairly torquey motors, or the New type SHS High torque would be even better, Tienly GT30K.. are all reasonably priced and are undoubtedly better motors.

Piston - The CA 19 tooth is fairly good, or if you can find one the SHS 19 tooth.

Hop Unit - Prowin M4 Hop unit, or Madbull. They have the more sturdy dials similar to the ICS but would not have any issues with nozzle compatibility.

MOSFET - Computer controled? Or just standard + reliable? I have one of those going if needed.

Other - At 3am im a bit fuzzy so might miss something. Prometheus Blue Hop Bucking - fantastic quality and seals very well. Also works perfectly with the r-hop patch as there is no hop sector to remove.

[henry1234]

Re: Sr-25 dmr

Thank you very much for your swift and detailed response, it's very helpful.

I have a few more questions though

- is a 6.03 barrel better for range and accuracy than a 6.01?
- I didn't pick the G&P motor out of any brand loyalty only that I thought it was the most powerful, and as I am aiming for almost instantaneous trigger response I though it would be a good one. Would a lonex provide this?
- the reason I didn't originally go for the SHS 19 tooth is the piston (teeth aside) is polycarbonate whereas the other one is aluminium, wouldn't a metal one be better for the power this will be firing at?

Thank you for all your help

[ChuckNorris56]

Re: Sr-25 dmr

You want to piston assembly to be as light as possible. This puts less stress on the motor, and the response will be a but faster also. I'll put it in these terms; Imagine kicking a football, then kicking a bowling ball with the same force.

As above, a Lonex A2 will satisfy all your needs. If you pair it with an 11.1 lipo and mosfet, you'll get the trigger response you're after.

As far as pistons go, I personally wouldn't use a polycarbonate piston. I find they're much too brittle and running them at DMR power levels, it will most likely crack. I'd use something of a stron polymer; Again, a lonex blue, or for a dmr build, possibly a lonex red, will suit you better. Don't let metal teeth put you off. People might tell you horror stories about stripping gearsets because they used metal toothed pistons. If the gearbox is set up properly, I.e proper shimming, AoE corrected, no over spin or pre-engagement, you shouldn't have any issues. I've been running the same gearset and metal toothed piston in a build of mine for years now with now issues.

Bear in mind this is only advice I'm giving you from experience. If you don feel confident doing these upgrades, maybe get a tech to help you out.

[sj_asc]

Re: Sr-25 dmr

Thank you very much for your swift and detailed response, it's very helpful.

I have a few more questions though

- is a 6.03 barrel better for range and accuracy than a 6.01?
- I didn't pick the G&P motor out of any brand loyalty only that I thought it was the most powerful, and as I am aiming for almost instantaneous trigger response I though it would be a good one. Would a lonex provide this?
- the reason I didn't originally go for the SHS 19 tooth is the piston (teeth aside) is polycarbonate whereas the other one is aluminium, wouldn't a metal one be better for the power this will be firing at?

Thank you for all your help

In theory, Yes the 6.03mm would be better for accuracy as you have to account for imperfections in BBs. Prometheus Inner barrels are also very easy to work with if you are installing one of Spikes r-hop patches, PDI's Hop window requires modification which can be difficult, they are also too deep, so modifications to the hop patch will need to be made to keep it at the correct resting height.

For trigger response - Although the Lonex A2 is relitively low in torque compared to it's competitors, it's still a great reliable motor and as said by chucknorris, would certainly provide you with the trigger response you are looking for if paired with an 11.1v.

Pistons - To take advantage of the full cylinder volume you would need to use the 19 tooth setup, the Lonex Blue or red are only the typical 16 tooth. If you were set on using these then you would have to either modify the intended SR25 gear set, or purchace a regular 16 tooth gear set.
The SHS piston body is durable and is made from glass fibre reinforced nylon. I would, as with all other FMR pistons epoxy the Rack in place to give it additional support, after this I would totally trust it in this setup over any other piston available.

[henry1234]

Re: Sr-25 dmr

Thanks for all the help guys,

So ill stick with the 6.03 barrel.
Ill move to the SHS 19 tooth piston.
Ill keep looking at motors though im definitely considering the lonex.

So
- any suggestions as to piston head?
- also does the sr-25 take a standard M4 nozzel?
- will sorboing the cylinder and fitting a more powerful spring reduce the cylinder volume to the point i shouldn't do it as it would have too low volume?

[MisterX]

Re: Sr-25 dmr

Erm I might be wrong but aren't most sites 450fps limit on .2 for dmr.

My modded g&g SR25 I purposivly made 440 to comply.

[richw]

Re: Sr-25 dmr

MisterX rises a very good point - most sites (in my experience) won't allow them above 425fps and some even 350fps, even with a mosfet limiting fire. If your local site allows what you are doing, many wont!

[henry1234]

Re: Sr-25 dmr

Thats a good point you raise, i was more focussing on the legality of the gun rather than the site limits.
Ill switch out the spring to a 425fps at 0.20g

[henry1234]

Re: Sr-25 dmr

Does sorboing reduce much fps?

[cheesenibbler]

Re: Sr-25 dmr

Sorboing can reduce FPS not by a huge degree though, 10-15fps in most cases as you have a shorter cylinder volume as the sorbo takes up a small part of it. I have had better results with the silent pistons, reason being is that the piston head is curved and hits a much larger surface area so the energy is dissapated better, If you are looking to make it quieter and last longer then go for the sorbo pad, if you are looking to get better energy dissapation then silent pistons are good.

Dont get a metal nozzle they are durable, but thats it, they do not seal better just due to the properties of metal over plastics get a plastic nozzle with o ring, as for the m4 nozzle fitting, I have tried two standard m4 nozzles (dream army and guarder) in my CA SR25 and neither are as long as the stock nozzle causing air leakage, so is worth checking this first.

As for metal toothed pistons in any set up, the metal toothed pistons work alright, but they tend to be glued into the polycarb or nylon reinforced pistons which means in some cases the metal teeth can separate from the piston or buckle/bend meaning they do not mess with the gear, I have had this happen in two guns I have had. Luckily nothing else was damaged, I epoxied the teeth back to the piston and it works fine now! One other thing to consider with the metal toothed pistons, you have a metal on metal contact on a high stress part, as opposed to a metal on nylon fibre contact where the contact point is the entire piston as opposed to the teeth alone, also it is going to be noisier then using a nylon fibre piston.

As for the gearset the SHS gearsets are very strong metals, however the middle gear (forgot what its called) is made in two parts, on high torque builds this gear can separate, again I have had this happen. Ill give you my current build that I have in my Classic Army AR10

Stock v2.5 7mm gearbox
Stock cylinder
Stock Classic army piston (very strong)
Element silent piston head/cylinder head set
Standard tappet plate dremelled to only allow semi auto
Stock nozzle w/o ring
Guarder M140 spring (circa 400 with stock 6.08mm barrel)
Stock bearing spring guide
Dream army trigger contacts (old ones burnt out)
Stock Gear set (very strong middle gear is a one piece instead of two piece)
Stock anti reversal latch
CYMA motor taken from a MP5K (very high torque, gonna be replaced by a SRC high torque gen 2 motor these motors are very torquey, low energy consumption and do not over heat from experience)
Madbull Steel Bull Barrel 509mm 6.03 inner diameter (circa 420fps with a 0.20g bb with the M140 spring)
Madbull hop up chamber
Madbull blue hop

The hop up rubber might be changed out to a firefly soft if I do not get good results with the madbull blue, but I will have to see this weekend at the big event!


One last point I dont honestly think there is much point to changing out the cylinder unless your stock cylinder has bad compression with the piston you are using and even then it is worthwhile seeing if another o ring is a better fit, then again it can save time with parts compatibility, the more parts you buy from the same manufacturer the more guarantee you have that those parts have good tolerances between them, hench using madbull barrel with madbull hop unit.