Changed hop rubber, drastic fps drop

[Enid_Puceflange]

Oh dear

My recently arrived (bought used)

Tm Scar was shooting a bit erratically, fps was spot on for a standard gun at just below 300 on a .2

but shots were pretty disappointing at range, like inaccurate at 40m

So, Ive changed the hop rubber

first couple of shots were at circa 270

Then it dropped to the 250’s

Wtf could I have done?

it all went back together no bother 

I used a G&G turquoise rubber 

Its a very tight fit on the barrel and into the hop unit

The only thing I can think of is a crap seal with the nozzle

But it also stopped feeding beebs into the hop unit

Given up for the evening as it took a good couple of hours to do it

so shall investigate during the week when my will power returns ?‍

View attachment 98614

 

[Rogerborg]

Basics first, did you replace whatever's tensioning the hop unit against the gearbox?

 

[Enid_Puceflange]

Yeah, I believe so? ?

i had it apart last week to see what condition the rubber was in

Everything was swimming in silicon oil, so I have managed to reassemble it and it fired better last week

Looks like I should just get it apart again and re assemble 

without a beer

Are they sensitive to different brands of rubber?

should I stick to a TM?

(the one fitted was laying around in a spares box)

 

[Nick G]

What state is the inner barrel in ? If the fps was consistant but the shots were 'scattering' I'd suspect that. Might need a good clean. 

 

[Enid_Puceflange]

Yeah,

My standard practice when a new arrival comes in is to remove the barrel and hop and give it all a good clean

Barrel was cleaned with isopropyl, the rubber in it when it arrived was swimming in silicon ?

So after cleaning all of that, it was placing out to 50m, but no real accuracy

(50% hit rate on a torso sized target)

I had hoped changing the rubber would help, but obviously I havent got something right whilst reassembling 

 

[Skara]

Might be the feed lip being too short, so when the nozzle goes into battery (fully forward), it doesn't seal anymore against said lip.

The opposite issue (lip being too long) is common on Maple Leaf rubbers, which causes feeding issues.

(it's why we have 450 thousand different nozzle lengths to choose from btw)

 

[Rogerborg]

I believe so?

There should be some sort of spring pushing the hop unit back against the gearbox. I mention this because they're easy to lose or forget when reassembling.

I only use Maple Leaf rubbers now because they have long feed lips so should get a good seal. If they mis-feed (tested by dropping BBs up the inverted feed tube to rule out magazine issues) you can trim away the bottom lip as necessary.

 

[Enid_Puceflange]

Thanks folks

i shall get back into it during the week

No components were left on the table, but may have put the spring and spacer in back 2 front 

 

[Enid_Puceflange]

Update:

Took it apart again earlier, discovered that in my laziness at disassembly, I had merely loosened the six screws at the hop end of the external barrel, to allow me to free the internal & hop unit

What I hadn’t realised is there is a tab on the side of the hop unit that slots inside the external barrel. So the hop unit was sitting maybe 1cm further back than it should have been. Therefore , it couldn’t feed as the nozzle was protruding into the hop assembly too far and stopping beebs from feeding

Doesn’t explain how the fcuk I managed to get 3/4 shots through it before it stopped feeding previously ??‍
 

Anyway, now shooting .2’s at mid 280’s fps

(was previously just below 300 with standard worn rubber)

Will fps climb a touch as the rubber beds in? ?

this was read with minimum hop applied 

 

[Madhouse]

Anyway, now shooting .2’s at mid 280’s fps

(was previously just below 300 with standard worn rubber)

Is the new hop rubber now hopping the weight you want out to a longer distance? That was the plan after all.

No idea if there's a 'bedding in' period for hop rubbers.

 

[Enid_Puceflange]

Not tested it on my ideal for it which will be .28’s

 

[Pollynator_bravo2]

I’ve had similar on bucking upgrades. I’d check to make sure the hop is aligned but if you have changed only the rubber then I’d say the only think that could be happening is a loss of air as the nozzle and bucking seal. Did the original look the same as the new one in terms of dimensions?

 

[Enid_Puceflange]

Yes, 
I made sure the bucking was as far forward as the original was to ensure it was getting a good seal with the nozzle

Maybe - hopefully, the nozzle and bucking just need to “bed in” together ??‍

 

[Stevobeavo]

Could be that as the old bucking was covered in silicon oil there was less resistance as the BB passed through it. 

 

[Enid_Puceflange]

Could be that as the old bucking was covered in silicon oil there was less resistance as the BB passed through it. 

Yeah, I suppose that’s a possibility.

The Chrono reading is what I would expect for a non tampered with TM anyway, so I’m comfy that it’s pretty healthy overall.

It was chucking .28’s down range last week no bother, just no good accuracy at long range. 
Hopefully the fresh rubber will see to that 

Fps wise, I’m not going to tamper with it any more

Im a firm believer in the TM fairy’s ? 

 

[Pollynator_bravo2]

Keep us updated as it’s a good lesson to learn from 

 

[Enid_Puceflange]

Keep us updated as it’s a good lesson to learn from 

Well, I replaced the old bucking with the fresh new one 

Totally dismantled the hop unit to remove any contamination etc and re assembled

The fresh rubber was an exceptionally tight fit, over the barrel and getting back into the hop unit.

My first testing of it was in the back garden (before the cold snap we just had)

So I would say it was roughly 8 deg outside - keeping in mind the rif had been stored in the house at circa 20deg just before testing

fps was good - still around 290

And it was lifting .20’s like a champ

My next outing to test it was on site, the day never got above freezing

Range was poor, maybe 30m and that was firing .28’s which is what I intend to use 

Accuracy was ABYSMAL 

I didn’t have .25’s with me to try slightly lighter

Im hoping that the freezing temperature was the reason for a lacklustre performance , causing the hop to remain quite hard and not give much Magnus effect? ??‍

 

[Davegolf]

Has this gearbox been removed/tech’d previously?

Some thing to look out for on the NGRS AR platform… the special long nozzle has a notch in the bottom at the front end, likewise at the back is a anti rotation ‘key’ which locates into the tappet plate to correctly orient the nozzles aforementioned notch.

If that isn’t in correctly you’ll have all sorts of crazy shiz.

I take it you’ve done all the usual;

Roll inner barrel on bench test if straight.

Inner barrel slides into outer without resistance. (Outer bent).

BB drop test through barrel.

BBs good, not swollen/deformed/old/cheap.

Bucking lips evenly seated/protruding from hop unit.

 

[Enid_Puceflange]

Has this gearbox been removed/tech’d previously?

Some thing to look out for on the NGRS AR platform… the special long nozzle has a notch in the bottom at the front end, likewise at the back is a anti rotation ‘key’ which locates into the tappet plate to correctly orient the nozzles aforementioned notch.

If that isn’t in correctly you’ll have all sorts of crazy shiz.

I take it you’ve done all the usual;

Roll inner barrel on bench test if straight.

Inner barrel slides into outer without resistance. (Outer bent).

BB drop test through barrel.

BBs good, not swollen/deformed/old/cheap.

Bucking lips evenly seated/protruding from hop unit.

Nope! ?

I never did any of the aforementioned inner barrel tests, so more stuff to look at next time it’s apart. ?

It does have a stainless barrel, and unless it’s different to the Scar L , I believe they come standard with a brass barrel installed?

The inner certainly went back into the outer without any fuss or hints anything was bent.

I dunno about anyone being inside the gearbox previously 

I’ll see if I can get any clues without letting the fairies out of their captivity ?

 

[Davegolf]

If you pull the upper off the gun, look at the front of the nozzle, there should be a half moon notch that points down toward the magazine.

PS looks like your TM didn’t get any fairy dust ATM ?