Adding Weight to Bolts to Reduce ROF?

I'm prtetty sure this has been discussed in the past but I'll bring it up again.

Two videos I've done recently of my MP40 (abs) and M3A1 Grease Gun show the models working at too high a rate of fire.

Slowing the action down would be more realistic, and as Mark said, would last longer so doubling the fun

Is it possible to add molten lead to zinc alloy bolts? What about filling cavities in plastic bolts with resin and lead shot for instance?
Would the increase in bolt weight slow the action down?
Would recoil springs cope?

Has anyone tried this out please?

Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by MadMike

Yes I too have read about this issue, but can't remember where. Anyway bear in mind that the blowback effect caps propduce is limited, so the bolt may become too heavy. I remember early Marushin factory assembled M16 versions with heavier bolt carrier, those models frequently jammed and could only function using two caps, so Marushin came up with a "light bolt carrier".
Maybe someone who has done that can help. If memory serves me I heard about Thompsons with slower rate of fire due to heavier bolts.
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Post by 8ace

That would be my Thompson bolts .

My custom Hudson bolts are a lot heavier that the original zinc ones which did slow the rate of fire down.

How much weight to slow the ROF down will depend on the gun as I noticed that Hudson bolts are light compared with their size(which would explain why they deform so easily) but Marushin and MGC bolts are heavier (possibly due to better less porous zinc).

The epoxy and lead shot technique should work well on plastic bolts. I have seen this stuff

http://www.aluminiumrepair.co.uk/html/hts-2000_technical_info.html

(which i thought might be good for zinc gun repair) might be an alternative to pure lead also general purpose lead/tin solder might be good to.

But all of this will put a strain on other components so perhaps adding rubber bump stops and other things to take the greater impact.

8ace

Hi 'Mike

Thanks for your reply. Yes you're quite right, too heavy a bolt may well prove too much for the cap and cartridge to blow back.
I guess that lead would be easy enough to take off again by applying some heat.

I wonder if anyone has tried it though and found the "ideal" weight to add to a Hudson Grease Gun's bolt?

A model maker told me about using resin and lead shot, he uses it to make ballast on model boats and it's an easy way to distribute weight where you need it. I wonder if a little added to a plastic MP40 bolt would work?
The abs MP40 is such a reliable firing gun but only when it's set up properly. One well greased cap, and clean det pin and chamber will produce great firing, but mess about too much with a second cap, or as I'm finding now, Mg powder and everything stops!

I fitted one of 8ace's steel bolts in my MGC Thompson. It fires really well but I couldn't tell you for sure how much slower it fires now. It'd be an idea to swap the bolt with a new zinc one and try them side by side wouldn't it?

The Marushin UZI I have has a relatively heavy bolt and carrier. The recoil spring is really strong. MGC P220 rounds work well in it, and it fires quite slowly too.

Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by Claymore

Adding weight to a plastic bolt may cause problems with the damaging barrels chambers etc especially on those models prone to break also i think you would have to add a lot of weight to slow down the rate of fire anyone who bought the mgc steel sten will know the bolt is very heavy and does slow the rate of fire considerably but 8Aces bolts tend to match the rate of fire of the original zinc.
I would say that maybe a stronger recoil spring would be the better way to go for slowing rate of fire if of course you can find a stronger spring, or maybe double up the spring and see if that works

Post by Mark

Cerwyn,

I added weight to my Hudson Thompson's bolt by melting and pouring in molten lead (a couple of bullets!) into the large cavity on it's side. I noted that afterwards the bolt was heavier and the gun cycled slower when i fired it. On the all zinc modelguns, i don't think that the addition of extra bolt weight will damage things too much. On the ABS/HW versions. i would add very little weight to them, just enough to make it a little heavier than it was originally and not go "overboard". The Hudson M3A1's bolt could perhaps use some wheel balancing weights (the stick on type) attached on the inside or some where else. Claymore, you do have a valid point there with your observation on the most of the modelguns lack of strength in key areas such as the chamber areas etc. Although, changing the spring tension might also cause some wear/damage problems as well.

So, i am not sure that there is really any proven method of slowing down the rate of fire on some of these modelguns without some experimentation.
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Wanted to buy: MGC Sten MK3....trades?(still)

MGC MP40 Marushin MP40 Nakata MP40 TRC MP40

Modelgun manufacturers, past and present:CMC,CAW,Hudson,HWS,LS,MGC(R.I.P. 1960's~2007) Marushin,Marui,Kokusai,Shoei,Tanaka and Western Arms

Plenty of food for thought there

8ace's point about adding rubber bump stops is one I remember discussing with one of the other forum members awhile back. We were talking about the effect of heavier steel bolts travelling backwards at speed. Is there a danger of the bolt slamming against the back of the receiver hard enough to damage it or would the Recoil Spring not compress enough to let the bolt travel that far back?
An extra piece of rubber wouldn't do any harm though just in case.

Claymore's idea of stronger springs is another good one, if you can find stronger springs! I wouldn't know where to start with that one, where could you walk into and buy a selection of springs from? Any ideas?
Adding Pre Load will stiffen a spring, but that reduces travel.
Packing a few washers behind the spring could be an easy way to experiment with that idea.

MGCUK's steel Sten does have a heavy bolt and fires at a lovely rate.Ít just sounds so "right". Watch Claymore and Kickbacks Full Auto Video to see the difference between the Sten and everything else.

Mark's note about adding lead from molten bullets is the one I read a long time back ! Wheel balancing weights are a good idea, they're easy enough to get hold of and inexpensive too.

After all that I wonder sometimes if I should be satisfied with the fact the modelgun actually works and not start thinking about modifications that might break it or stop it firing!

Thanks very much for everybody's input, I'll be searching for washers and wheel weights this week

Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by Doc

It may be of interest that the early steel M3A1 has a bolt weighing 615g and the ABS one is only 215g.

DOC
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We need guns...lots of guns....

.... well anything mechanical that can kill people is interesting ...

That's a whopping big difference isn't it DOC?
I don't think somehow the all abs M3A1 will take kindly to having a bolt that heavy thrashing about inside it.
You mentioned a little while ago that these Hudson M3s can blow their barrels off... I've visions of barrels flying off quickly followed by a bolt...

Might be best to be content with the fact it fires no matter how fast

Thanks very much
Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by Doc

Sure is ,

anyone done a test to see how far the M3A1 ABS bolt travels rearwards ?

I can see it needs to travel back at least 3 inches to engage the sear however in total it can travel rearwards 5 inches before it bottoms out .
what Im thinking is that IF (big if) the standard bolt can go a little further rearwards then you could fit either shorter or softer springs and this may allow a slower ROF .

All depends how far the bolt comes back as is .
(you`d have to look slow mo of a some vid of the ejection port )

The Steel sten does have a heavy bolt but also has a relatively soft recoil spring so that bolt is coming all the way back .

DOC
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We need guns...lots of guns....

.... well anything mechanical that can kill people is interesting ...

Thanks DOC

That's interesting reading, so by softeningthe recoil spring(s) ROF could be slowed down...
I was thinking of it in reverse. So a softer spring might allow the bolt to travel further rearwards if there's room, as the spring might compress more, and then return forwards again but as it's travelling both further and slower as the spring isn't pushing it as hard the ROF slows down?

Stiffer springs or pre-loaded ones would give the bolt a shorter stroke and shove it forwards harder so could even increase the ROF then?

I wonder if softer springs would be strong enough to strip rounds from the magazine? One of my other models had a tendency for the bolt to stop over the magazine, mid stroke as it were. The recoil spring had worn out so wasn't able to push the bolt and round past the feed lips.
A new spring cured it instantly.

Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by Doc

Its all about the correct balance , I think these modelgun manufacturers spend quite some time getting the balance between recoil spring / bolt weight / cartridge size /power just right .

It took the same amount of fiddling to get my BF MP40 just slow enough ...I see loads of guys using them and they run as if its a pesh ....waay to expensive

DOC
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We need guns...lots of guns....

.... well anything mechanical that can kill people is interesting ...

Doc wrote:

Its all about the correct balance , I think these modelgun manufacturers spend quite some time getting the balance between recoil spring / bolt weight / cartridge size /power just right .
It took the same amount of fiddling to get my BF MP40 just slow enough ...I see loads of guys using them and they run as if its a pesh ....waay to expensive :wink:
DOC

And a fine balance it is too. Cartridge power is suprisingly high when you consider that a single 7mm cap will blowback and propel your Sten's heavy bolt back and forth with no trouble.
Adding a 5mm cap on top of a 7 in the abs MP40 proved disastrous really as the bolt got kicked back so hard it tripped over itself. Must've been way too strong for the recoil spring to cope.
It would be nice to slow the M3A1 down a tad though, but not if it's going to bugger it up completely!
Cerwyn

Ah, I was just wondering about adding weight inside my Maru. MP40 bolt. I was thinking of using something not so permanent, like solder. Instead of melting it, I would just cut lengths of it and put a stick of it in the bolt, test it, and then try another until I got the desired effect. I might even try to pack it a bit with some foam so it doesn't bounce around inside the bolt.

We must be on the same wavelength I was thinking about that hollow abs bolt this morning and wondered if some weight might slow down the rate of fire
There's definately plenty of room in there and it could well work.

I was just about to send you the links to the "Modifications" thread I started some time ago, but I see you've already found it

Your idea of strips of solder wedged in with some foam sounds good. Self Adhesive Balance weights for Car and Motorcycle road wheels are an option too and another idea I've heard of is to add buck shot to epoxy resin and then pour the mix in. Once it's set it won't move around.

The MGC model's all metal so the bolt's heavier and fires at a nice, more realistic rate. Finding the right balance between recoil spring strength and bolt weight will be the challenge as has been discussed further up this thread
I have still to find time to play about with my Marushin MP40, I have a spare recoil spring ready for cutting here but I'm not convinced that just a shorter spring will slow the rate down.

Be sure to let us know how you get on please

By the way, try Francky for more cartridges... he's very good with prices and his service is superb... http://franckys-modelgun.com/

Well, let's see if I can get the old photobucket kicking. I made the heavy vs light video. It is semi-scientific, and can be debated for accuracy.

If I wasn't lazy, (or if I would have thought of it at the time....) I should have used the same gun but switched the bolts.
It's possible one spring has more tension than the other, or the bolt slides better in one receiver than the other, etc.

BUT..... the fact the lighter bolt actually travelled further (I'm guessing around 2" more) than the heavy bolt, YET the heavy bolt took more time to travel the shorter distance it travelled, should say something too.

By the distance the heavy bolt travelled, I would say adding more weight would start creating problems. I'll have to find my scale and weigh each bolt and report back.

Heavy bolt: 168 g
stock: 145 g

No damage yet....

In the back of my mind I know the designers of these guns at Marushin were no fools, and I have to wonder what not-leaving-well-enough-alone can do. I am a notorious tinkerer and have a "even the best designs can be improved" philosophy.. hehe There's my struggle...

I am in the middle of another test to clarify the results in this video. I've already shot a video of the heavy bolt firing 3 rounds in full auto. (both MP40s are working perfectly now! ) I figured there were too many what-if's by only shooting single rounds and in two separate guns. Now that I shot the heavy bolt MP40 footage, I will put the stock bolt in the same gun, shoot the three rounds and that should clear up some of the questions. I hope I'm not going to start boring people with all these drawn out tests hehe.

I've confused myself regarding slowing down the videos multiple times, and how to convert the figures back to real time numbers. It's not so much the math, but whether WMM (Win Mov. Maker) converts the footage accurately 2x, 4x, 8x etc. Perhaps you can help clear the confusion..

Real time is 30 FPS
I film at 300, so I'm starting out 10x normal speed
Then I half-speed it three instances: 20x -- 40x -- 80x
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Now, let's say we take the cycle time of 12.80 (at 80x speed-- IF 80 is the correct number at all)
Divide 12.80 by 80= .16 seconds
That is where I have to wonder if my numbers all jive or not.
.16 seconds seems too quick.

I need to film a stopwatch to check my figures I think.

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Yes, it's wild seeing how everything seems to flop around. The barrel is wagging, the receiver seems to be flexing in the middle, and the magazine looks like a clanger on a bell! I thought I had that pretty snug too!
I saw a video on a guy shooting a pellet gun with a steel barrel, and that barrel wagged around quite a bit.

Ok, did the stopwatch calibration test. I won't bore you with the video from it. lol
I can tell you it is more exciting than my "Watching Grass Grow" mini-series.


At 300 FPS, it took 50 seconds for the second hand to move 5 seconds. 10x is correct

I "slow down, half'd" it 3 times. So the equation is 10 x 2 x 2 x 2= 80
It took 400 seconds for the second hand to move 5 seconds.
So it seems the camera is accurate, and WMM is actually doubling the time correctly for each "slow down, half".

400 divided by 80= 5 seconds real time.
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Sometime I will try to take a slow motion video, and then speed it up to show what it looks in real time.
I now see that I was off a little. At 10x, if I double the speed THREE times, I end up at 1.25x and not 1x.
Close enough I guess.

I'm guessing that you guys calculate your cyclic rate by seeing how long it takes to fire 30 rounds.
If it takes 10 seconds, your ROF is 180 RPM.

60 divided by time it takes to fire x number of rounds fired.

Does that seem correct?

Not the huge numbers we wanted to see, but a change nonetheless.

Heavier Bolt Final Video

Excellent tests and even better to see what is going on and I reckon you calculations are spot on.

8ace

In the first video I said I added "a couple ounces", but it was only 0.8 ounces. I think we all knew more weight would slow the ROF down though. The change is pretty minor and would be difficult to notice in your gun I think. I've seen some videos of real MP40s and it seems their ROF is around 3-400 RPM. The information I found said they fire at 500, so I don't know what to believe.