FPS/Joule Drop When Hop is Set

[Cr0-Magnon]

My G&G MBR556 is setup as a DMR and shoots 2.1J on a 0.20g with no hop applied. However when setting the hop for 0.40g's it drops to 1.5J and set to 0.36's, it sits around the 1.6J mark. I've just put a new M140 spring in it and not keen on the idea of going up to an M150 to achieve 1.8J on a 0.40g.

The stock inner barrel is 463mm but I'm using a 455mm ZCl in combination with the stock hop unit, a 60 degree maple leaf macaron & omega nub.

I appreciate a drop in FPS/Joule output when setting hop is normal but think going from essentially 475fps to 400 with the hop set, is quite an extreme case.

Is there anything I can do to reduce the effect?

 

[Salamanca]

Hello,

A change in bucking could help. A high quality bucking will provide more hop for less hop pressure. Also barrel length is crucial in DMRs, as you need to get a good cylinder to barrel ratio, as you are using heavy bbs that take longer to leave the barrel than say .25s.  

You may need to shorten the inner barrel length. I have built several DMRs using 140 springs and a 360 to 370mm barrel length, was what I found worked best with a V2 non-ported cylinder, using 0.43s. I recently built a G&P SR25 DMR which has a longer V2.5 cylinder, and that was able to feed a 407mm barrel, but I would not have gone any further in length. Your rifle is able to shoot at 2.1J on 0.20s because they leave the barrel before the 'cylinder air charge' is spent. The .40s and .36s you are using are passing through the final section of the barrel by momentum alone, while suffering from the friction of the barrel. Hope this helps.

 

[Rogerborg]

Eh, it is what it be.  My G36 (with an adjustable spring tensioner) shoots 1.2J on 0.2g but bang on 1J with 0.28g, at maximum pre-tension, which isn't far off of the difference you're seeing.  This is OK, since I can dial it down to get under 1.1J with 0.2g for CQB, and I'd rather keep it usable indoors with 0.2g and outdoors with 0.28g without spring swapping. 

My DMR is shooting 1.8J with 0.32g.  I couldn't even tell you what it's shooting with 0.2g, or any other weight, because... well, why would I care?  I've tuned it for the BB weight that I use with it.  That's what you'll have to do too, one way or another.

60 degrees might be a bit grabby for 0.4g, have you considered 70?  Looks like it's already got a rotary hop, I wouldn't change that.  Air seal spot on?

 

[Cr0-Magnon]

A change in bucking could help. A high quality bucking will provide more hop for less hop pressure. 

Which bucking do you use in your DMR? I would have thought a Maple Leaf was good quality.

You may need to shorten the inner barrel length.

I was thinking this too. So you reckon around 370mm?

Eh, it is what it be.

Oh yeah totally haha, one of my favourite sites has a 1.68J limit. So loading it up with 0.36's and I'm spot-on. Half of this is just for the sake of tinkering/tuning. Not like I'm going to be massively suffering, even at 1.88J sites.

60 degrees might be a bit grabby for 0.4g, have you considered 70?  Looks like it's already got a rotary hop, I wouldn't change that.

I've just changed the 70 for a 60, so can easily swap back. Yes the stock hop unit is rotary.

Air seal spot on?

I guess so, it was upgraded fresh out of the box by Luke this year.

 

[ak2m4]

Out of interest what type of cylinder does it come with, you mentioned the stock inner barrel is 463mm so I'm assuming closed?

 

[Fatboy40]

I'm deferring to more knowledgeable people, however trying to effectively hop double the weight of BB you acknowledge that you'll lose power, and it can be quite a bit.

For myself and my DMR build I've also used the blunt instrument that is a stronger spring to increase power, but I know that this is like pushing a car to the red line in that it will put substantially more strain on the gearbox and motor.

My goal now is to hop the BB as effectively as possible, and by doing this I don't have to use crazy springs etc.

I'm using a Maple Leaf AEG inner barrel and MR / Maximum Range bucking in both my DMR and AEG (one with a Omega nub), effectively giving me an off the shelf r-hop as with a Macaron you have to apply too much pressure on the patch and lose all that power. I'd say move away from the Macaron and go to an MR or similar, but this may mean a new inner barrel otherwise the bucking's patch may not sit well in the cut-out.

A recent revelation to me those has been a Maxx Model M4A-PRO hop unit and their 21.25mm nozzle. I know it's not cheap but I just fitted these to my AEG where I was running a ZCI rotary plastic hop unit, where I could never get the amount of hop spot on and was losing too much power. The Maxx unit and nozzle magically gave me an extra 15 FPS on 0.28g BBs, Maxx say there may be an increase but this was brilliant and I've gone down now as well from an M110 spring back to an M100. Testing it in a big open space last night though the change to the hoppping was stunning, where I'm using their solid metal concave nub, and I could apply more than enough hop with only a light pressure.

If on my AEG, shooting 0.28g BBs with the ML barrel and ML MR bucking, the Maxx hop unit + nub + nozzle can make that much difference I can't wait to have the money to buy one for my DMR (I shoot 0.40g BBs with an M130 spring, 4/5 cylinder and 410mm inner barrel).

 

[Sneaky]

I would recommend an R-hop for a DMR build, works well and is very consistent. Does the air nozzle has an internal o ring?

There should not be that much of an fps reduction with hop on, if you are having a 75 fps drop. 

 

[Cr0-Magnon]

Out of interest what type of cylinder does it come with, you mentioned the stock inner barrel is 463mm so I'm assuming closed?

Annoyingly I've lost the videos Luke sent me, believe it's the stock one though. Believe it is closed. I've put an order in with you for a ZCI hop chamber and 380mm barrel. Thinking I should add a 70 degree MR hop to that too ? 

 

[Lozart]

Annoyingly I've lost the videos Luke sent me, believe it's the stock one though. Believe it is closed. I've put an order in with you for a ZCI hop chamber and 380mm barrel. Thinking I should add a 70 degree MR hop to that too ? 

I've got a feeling the hop window in the ZCI barrel is a touch too short for the MR rubber (happy to be corrected though @ak2m4)

 

[ak2m4]

@Cr0-Magnon When Luke did the work did he configure it as a DMR originally?  For standard builds he does a lot short-stroking.  I don't know much about those G2 gearboxes.

@Lozart From memory I think it's the other way round, the zci hop window is around 1.5mm longer for the MR patch, but it's never really caused me an issue on testing, however every single brand will react differently. 

 

[Cr0-Magnon]

I obviously need to go back and watch this, my gun is featured in this video: