GTA
Gateway To Product Reviews => Air Gun Mods and Tunes => : speedturtle May 02, 2009, 08:41:46 AM
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I've been toying with the idea of shortening the barrel length of my Crosman G-1 Extreme but still waiting for some additional info from some research that I'm doing. Finally upon convincing myself that I have a definite plan of action I decided to shorten the barrel of my G-1.
I was reviewing the article made by Varming Al regarding barrel tuning using a weighted tuner unit near the muzzle of the barrel to affect the timing of bullet exit at the muzzle and there was this little link (actually a very funny one) about listening to your barrel for vibration and ringing (excessive vibration similar to plucking a string of a guitar). being familiar with audio and sound I decided to see if there is a merit to what it wants to explain. I like the author's preface - "This writer is a reporter, pure and simple (minded). I am no physicist, engineer, metallurgist or scientific type of any sort. Any similarity between the content of this article and scientific accuracy must be attributed to the work of others more qualified. Hopefully, however, some of their findings, valid or invalid, are accurately reflected herein. That said, settle back and, hopefully, you, too, will enjoy an enlightening chuckle or two. That is the intent." I like that. Just a sort of a very enthusiastic rifle fanatic that somehow "discovered" something that affected his curiosity and pursued it.
I am no engineer myself too, just a regular guy who never stops trying to figure out if my idea works or not. Being involved in audio before I thought "probably I can use some of the ideas of room tuning to airgunning. Hahaha!" So this idea came into work! Here goes.
Hoke Kerns (the author of the article from this site: http://www.precisionshooting.com/psm_2007_09_issue.html) worked with a guy by the call-sign "COLT .45" and experimented on tuning their rifle barrel by "listening" to its resonance sound using just a wooden mallet to excite the barrel when struck and listen to it using a stethoscope placed at the surface of the stock. You would hear 3 sounds actually, the first is a "Bong" with a small tail of resonance into it, the second is a "BOOONNNGGG" that has a more pronounced longer resonance tail of sound into it, and the last one would be the "THUD" a solid type of sound that has a sharp note but without the ringing tail associated with the bong and the booonnnnggg sound. Now their idea was to "move" the THUD sound very near or almost at the muzzle by adding a weight tuner (with a variable weight system). The idea actually works ( I can attest to this).
To mimic or replicate their analysis I decided to just tap the barrel of my Crosman G-1 Extreme (without the muzzle brake attached) using a wooden dowel and listen to it with the diaphgram of the stethoscope touching the synthetic stock at the C.O.G. area of the stock. I can hear the Bong, Bong sound all through the length of the barrel but suddenly shifts to a THUD at around 14" - 14.625" from the end of the breech. I found that interesting. I noted that on a piece of paper and then started to add weights on the end of the barrel using the assorted sockets that I have (from a socket wrench set). I don't have a weighing machine so I just used the heaviest of the sockets and use some rubber bands to put them together and hang them tight to the end of the barrel. What I have noticed upon hanging the added weight was that the "THUD" sound did actually shifted "forward" near the muzzle of the barrel by about 0.75" (similar to what Hoke and Colt .45 noticed too). In my G-1's case this "dead zone" area of THUD sound is about 5/8" in range. After that 5/8" zone it starts to ring again and have that BONG sound.
My G-1 is already too heavy (even without the Leapers 3-9X50 scope) since Crosman makes their airguns mostly with all metal parts so I don't want to gain some additional accuracy into it by adding more weight using a weighted tuner at the muzzle. What I want to find out is that If this "zone of neutrality" exhibited by the THUD area be the right spot to cut the barrel and "therefore" (this is just my analysis) bringing the dead zone at the crown area. Instead of using weight I just "shifted" the dead zone by shortening the barrel (in a theoretical point of view).
So, now I know that the barrel of my G-1 has its dead zone from 14" - 14.625" from the breech, all I need to find out now is exactly "where" will I cut my barrel at this dead zone? So Off I go to my old ideas about room acoustics and come up with this analogy:
"we excite a room with multiple frequencies by putting a subwoofer at a room's corner. It does not matter where, any corner will do (even at the corner ceiling! LOL!). Then to know where you can locate or position your listening is to divide the room into odd multiples like (Length divided by 5, by 7 by 9, by 11, etc....) and plot them in the room using masking tapes. Then by actually moving your chair very near these odd lines or intersections you can actually find a sweet spot wherein most of the frist modal room resonances is actually not that strong" (just like what hoke and colt .45 found out in a barrel of a rifle). Sound like a voodoo science eh? Well, you can think of it like that but in reality there is some truth into it. In audio the axial modes of the rooms resonance are the most notorious to control and they affect the overall frequency response of your speakers (regardless of brand except other design like planar speakers) wildly! The rememdy that we do before room acoustical tuning is to position the listening chair to a sweet spot and from there work on refining the sound by adding some acoustic panels into the room. (Enough of the room acoustics stuff. Hehehe).
Applying this to my G-1's barrel, I am "assuming" that this THUD area (zone of neutrality) would also mean that the main axial modes of resonances of the barrel will be exhibiting the least amount of vertical deflection (a form a first-order resonance mode) and so I'm convinced that this is a good spot to cut my barrel. Now I already know that I can but my barrel anywhere within 14" - 14.625" but where? What I did was to divide the original barrel length of 18.5" (this is the G-1's original barrel length) with odd numbers and find out which among them is within this zone of neutrality area. Here is what I came up:
18.5 inches divided by 9 multiplied by 7 = 14.39"
18.5 inches divided by 13 multiplied by 10 = 14.23"
I opted for the 14.23" length. I don't have the midwayUSA's tool to totally make the crown perpendicular to the barrel so what I have used it to buy this 90 degree metal brackets that I use to attach wooden planks on a cabinet. Using just a regular hacksaw I cut the barrel. That's it. Now off I go to clean it up using a metal file and a 280 grit to clean the new crown. I used a metal countersunk bit to finish the crown and I'm satisfied with the results.
Here are the pics of the final outcome.
The advantages? Well, for one thing I like the way the weight is balanced now on my G-1. I did not notice any additional recoil from shortening the barrel. In my personal experience I kinda like it more now that it is less weight heavy. The only downside is a more pronounced cocking effort but I can live with that. Accuracy? Again in my own personal opinion and experience I gained an additional tighter grouping from my shortened G-1. That's all folks. Hehehe.
Speedturtle
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Very interesting Willie,....did you replace the muzzle break back on it after you chopped the barrel? How did that effect the acoustical "thud/dead"-zone over all. If so,...how did the extra "post-chop" weight of the MB enhance or reduce the harmonic balance of the barrel itself? Do you think this might shift the T/D-zone farther out or in if the barrel was weighted with the same weight as the MB? I think this is a very thought provoking thread,...and I'm glad you've shared your ideas and theory's with us. Got me thinking for sure!!!! tjk
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Yeah, I did installed back the original muzzle brake after some test firing of the shortened barrel. But I have a problem, the barrel of my G-1 is not totally concentric on the outside diameter (probably at the I.D. too) so putting back the original muzzle brake all the way in is not possible. What happened was that I came up with about 3" of internal hollow chamber for the trapped compressed air to affect the deflection of the pellet upon exit at the muzzle. I did not have any horizontal deflection, all was on the vertical plane (as explained by varmint al and confirmed by Alan Fisher and Hoke) so that means that the barrel resonance is indeed a 1st-order harmonic mode (an audio terminology). My P.O.I. shifted downward by about 1/4" which is acceptable. I guess that new hollow chamber did affect the pellet's P.O.I. but the G-1 is shooting consistently in the same direction. If there is any inconsistency the culprit would be my hold consistency but not of the gun itself.
What I did was to design my own muzzle brake using a PVC pipe and some uneven layers of chambers inside to reduce resonances within the chambers. The final new muzzle brake was about 7" in internal length and with an additional 2.5" extension to center the brake on the barrel. I did some new testings with the new muzzle brake on and it is more consistent than using the original brake. What I meant was that it really producing more tighter groupings. At 10 yards indoors all 5 shots are placed inside a 1/4" area. I just copied my old muzzle brake design but used a smaller O.D. of PVC pipe to make it look less bulky.
TJK, I did not get your question correctly so I can't give you a definite answer for now. If you are asking if the additional weight of the muzzle brake when attached further enhanced the harmonic balance or characteristic of the barrel and the gun then I would say "probably". Even without the muzzle brake after the barrel shortening the G-1 has already exhibited an improvement in its P.O.I. groupings so the only difference that I "saw" after installing the muzzle brake is a difference in the vertical plane location of the P.O.I's. Hope this answers your question.
I learned a lot of things from this forum so I am just returning the favor. I'm still a newbie, just a regular guy that is really quite curious about this very nice hobby. My next project would be to design and make my own wood stock based on the Feinwerkbau stock and incorporate it on the G-1. My hands are small and my finger's reach is limited so I really need a different stock to help me pull the trigger with better control. The problem is I have a very very limited skill when it comes to woodcarving and I don't have any tools yet so it will take some time. Hehehe.
Speedturtle. :)
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So how is this going to affect it's accuracy at longer distances?
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Haven't had the opportunity to shoot it greater than 15 yards so far. But at 15 yards it is still exhibiting good groupings. I just took the courage to fire some test shots through a window out to a nearby tree at 15 yards and test my groupings. It is not that hold sensitive for sure so I would assume better groupings from her up to 30 yards.
Speedturtle.
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Schmidty25, I have some good news from you - I was able to test my Home tuned and chopped Crosman G-1 Extreme this afternoon at my friend's farmhouse. at 20 yards it is exhibiting nice tight gorupings (all inside a 1/2" circle). I was shooting sycamore seeds (diameter varies from 1" - 1.25") from 20 - 40 yards and still hitting my targets. Then my friend came from picking up his kids from school, he took out this Daisy Powerline 1000S and we start some test firing. All of a sudden he spotted a small bird (I don't know what bird it was but he said that it's about 5 inches in height and small (i guess he already shot one before) at about 60 - 65 yards and at angle of around 60 degrees upward. He asked me to shoot it instead of him so he can confirm if the chopped barrel is still accurate. I took the shot. I aimed about 1 mildot below the head (since I'm shooting inclined or uphill) using CPHP pellet. When I fired the shot we saw the little bird spiraled downward. We tried to recover it but since it is already getting dark we were not able to find the remains. I believe I did not lost any accuracy in the G-1 when I chopped my barrel but somehow "gained" some more.
The G-1 by the way was sight-in at 16 yards using both CPHP and the Gamo Match (the difference in the P.O.I. is only about 1/8" from the result taken using ChairgunPro software.
Hope this helps.
Speedturtle.
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Thats really interesting. Out of curiousity have you re tested where the harmonic point is again (the THUD point)? especially with the muzzle break on?
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Yes, the "THUD" point actually shifted a little bit (because now the barrel is seeing a different total length so technically the harmonics also changes). But arriving at a certain conclusion based on my experiment it still made my G-1 a good shooting airgun with or without a muzzle brake attached. Attaching a muzzle brake just makes it more stable. A very plus factor after the cutting or shortening of the barrel is that I can use different pellets that does not deviate much on their grain weight and still group pretty well. Case in point, before the cutting my G-1 doesn't like any wadcutters at all. I'd be lucky if I can shoot them at about 3/4" near the bulls area but after the cutting these wadcutters can lilterally go inside a 1/2" circle at the bulls perimeter (all shots are taken from a 10 yards distance indoor). Without all the technical explanations I believe that the barrel's harmonics has definitely changed and for me it affected the G-1 positively. The sweet spot of my hold has changed too but that is already a given.
Hope this helps your curiosity.
Speedturtle
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I was just thinking about your little experiment and it seems to me that the "THUD" point is a node in the natural harmonics of the barrel. If you could make it so that point is the crown of the barrel, there would be next to no vibration affecting the exiting pellet. Obviously since the node can't be an end you would need a muzzle break to be part of the equation such that when the break is attached, the "THUD" point is exactly where the crown is. Seems like it would be possible to make this happen as I believe you could move the node by either shortening the barrel (as in your case) or lengthening or weighting the muzzle break. I am curious where the node is now on your gun in relation to the muzzle crown when the muzzle break is attached, and what you think of my little theory.
Kimball
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pyrophreek - 6/16/2009 11:58 AM
I was just thinking about your little experiment and it seems to me that the "THUD" point is a node in the natural harmonics of the barrel. If you could make it so that point is the crown of the barrel, there would be next to no vibration affecting the exiting pellet. Obviously since the node can't be an end you would need a muzzle break to be part of the equation such that when the break is attached, the "THUD" point is exactly where the crown is. Seems like it would be possible to make this happen as I believe you could move the node by either shortening the barrel (as in your case) or lengthening or weighting the muzzle break. I am curious where the node is now on your gun in relation to the muzzle crown when the muzzle break is attached, and what you think of my little theory.
Kimball
There is a ring of truth on your idea Kimball. The main objective of the experiment is to find out if "by locating where this "THUD" zone is on the barrel (it is a zone and not a point that can spread out from 1/2" to 3/4" in range) that I could somehow correlate it to the location on the barrel where the least "DEFLECTION" on the vertical displacement of the barrel is when a pellet is fired through it. Your idea was already proven by Hoke and Colt 45 but it was used on a rimfire type of rifle where there is just a single back recoil from the bullet fired. It is a different issue with an airgun (specifically a springer) because we are dealing with two recoils that really affects the accuracy of the airgun since the lock time is very much affected by the forward and backward recoil of the piston spring.
In my personal experience I still find the idea workable since I only have one airgun right now and it is this G-1 that I shortened. The thud zone is now about 1/2" from the crown compared to 4.27" from the crown before cutting. For me that is a big improvement! Also, shortening the barrel is just one of ways that you can shorten a gun's LOCK TIME. If you want to really become accurate with this then yes - adding some more weight at the end of the barrel (crown side) can really put the "THUD" zone technically at the crown itself. I did not do that anymore since my G-1 is already heavy because of that Leapers 3-9X50 scope that I installed! My main goal in doing this experiment is twofold and I have achieved that - that is 1. to take out some weight from the airgun and 2. shorten the airgun's lock time to gain some more consistency.
The last thing that I did to confirm my observation and analysis is to use a bench rest to test my G-1. Even without a muzzle brake attached the G-1 can make a 1/4" grouping at 10 yards indoor. With the shroud that I designed (a full tube cover for the barrel) the G-1 is just shooting just the same. With a very consistent hold I can put 5 shots on one hole using the rest. If your airgun is already exhibiting a 1/4" or even smaller grouping at 10 yards then you don't have to do any barrel shortening anymore. That grouping is already accurate for me for a springer. If you're using a PCP then a single-hole is not unusual even at 15 yards. It's just an experiment for me and I like the outcome of it. The funny thing though is that I did it on my own and yet when I read some posts of other members from here at GTA and at Airgunone.com that most of them shortened their G-1 also from 14" to 14.5" length and still shooting great! If that is just pure coincidence I find it a very good coincidence!
You can always test your ideas yourself Kimball. All that you need is the determination to do it and learn from the experience.
Speedturtle