GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: Kinetic45^ on October 13, 2019, 01:01:43 AM
-
The chamber of AirForce Texan rifles is too short.
For other DIYers here’s some detailed explanations how I fixed that.
Most bullets barely enter the barrel by finger force and since there is no bolt handle and/or probe to act as a force multiplier to help engrave the bullet into the rifling like on other styles of airgun you push in till it hurts your finger tips... and that’s just not good enough.
And so a great deal of the bullet usually hangs out into the ‘loading tray’ of the barrel. Hard to get consistency that way and consistency is the biggest key to accuracy.
This adversely effects accuracy because the bullet frequently ends up slightly cocked (usually the tail of the bullet is raised slightly in my experience) and when launched/fired it is swagged down into the rifling lopsided. Who cares if your only shooting 50 yards but if your shooting for precision or long range it MAKES A DIFFERENCE.
On both rifles I used a variety of bullets (I most commonly shoot) to test the depth they could be inserted with finger pressure and determined that the .257 needed 0.175 deeper and the .457 needed 0.200 deeper (see Pics of bullets in the chamber, before and after on the .257).
Note: I started out with the criteria of at least one caliber of bore contact as a minimum length to ensure a ‘squarely aligned’ bullet in the bore. Since most of the slugs I fire are fairly heavy/long the actual depth I would be cutting would be more than that which was even better. I wanted most of the bullet(s) to be in the bore for my rifles.
So I decided to make the chamber longer on both my .257 and .457 Texans.
I used throating reamers from:
https://www.reamerrentals.com/SearchResults.asp?Cat=25 (https://www.reamerrentals.com/SearchResults.asp?Cat=25)
I ordered them online and USPS delivered them in a Flat Rate box in two days. You get them for three business days but you can pay extra and get them for 14 days if you need. I also got the ‘insurance’ in case I accidentally chipped or broke one although with my experience it was unlikely. I expected the job to be done in one day and only take an hour total for both guns… and that included pulling the barrels out of the guns so did not need more days. It took 10 times longer to write this up than to do the job on both rifles combined.
Technically a cautious person could manually hand ream their chamber with a tap wrench T-handle, the reamer ends are 3/8” square ended. I would NOT use a crescent wrench for this as it would put side force on the reamer and cut the barrel wrong or even break the reamer!!! Use of a T wrench keeps the side forces minimized. You just have to take TINY bites as you feed forward and watch how deep you go… stopping a couple of times to extract the reamer, clean out the chips in the bore and test fit your bullet till it goes as deep as you want. Of course you would want to keep it VERY straight with no side force but the reamers are live piloted and do a pretty good job all by themselves. You would find that it cuts the rifling out so easy it only needs thumb n fingers on the T-wrench, not a gorilla grip turning the reamer.
LOTS of lubricant in the barrel and on the tap is NECESSARY! I use TapMagic myself.
I however have a small lathe, so used that to center everything instead of doing it manually. BUT, I NEVER TURNED IT ON!
First I GENTLY test fit by hand the reamer into the barrel, rotating with my fingers as I slid it in deeper to see when the reamer would touch the existing rifling lead in the barrel. I then measured with a caliper (from the end of the barrel to the ends of the flutes on the reamer) that to establish a datum point. Yes I might be off .002 or even as much as .005 but in this case the exact depth of the chamber extended is not THAT critical like headspacing a powder burner chamber. The depth I’m extending the throat is not based on only one bullets exact chambering need but is an average of different bullets length/profile for ‘best fit for all’… so I figure there is a +- of 0.010 anyway for ‘good enough’. Its why it could be done by hand too, not that much depth precision needed.
I put the barrel into the 3jaw chuck and instead of using a live center in the tailstock, just used a drill chuck. The intent was to hold the reamer non-rotating and I would turn the chuck holding the barrel by hand, SLOWLY feeding the tailstock forward a few thousands at a time.
I ran the reamer in till the datum distance I’d measured earlier was close, then with my left hand started turning the chuck about one revolution every 15 seconds and while feeding the tailstock forward at about a .01 at a time simultaniously, turning the lathe chuck a bit then advance the reamer another thousandth or so. In a minuet I could feel the reamer was taking a bite with just a bit of drag turning the lathe chuck with my hand.
I fed the tailstock forward about 90% of the depth I’d determined and pulled the reamer back, blew out the chips in the barrel and test fit a bullet. I saw that I was very close to the desired seating depth so used a cleaning rod to bump the bullet back out and reinserted the reamer forward… BEING VERY CAREFUL to approach the cutting depth slowly again so I did not take a huge bite and ruin the barrel or chip the reamer cutting edges. It only took me one test fit on the .257 barrel but it took two test fits on the .457 to get the ‘perfect’ depth I wanted.
Doing this the maxims of “Haste makes waste” or “Measure twice, cut once” are good to follow for a successful/satisfactory job.
I am NOT a photographer, just using my cell phone camera and resizing to fit the forum so I did the best I could with these photos.
-
Finally someone doing something to that nice feature af has made.
Nice job, interesting to see the results.
Marko
-
NICE JOB.
It is a well know fact in PG circles that matching bullets to chambers via seating and/or throating is one way to increase accuracy...I think that fact is now starting to bleed in to AG'ing.
Matching pellet to bore/grooves/weigh(length) is the favored way to increasing accuracy in AG's but for those that want the highest accuracy DOING BOTH has advantages...NOT ALWAYS...but more often than not.
My 357 Herc's chamber is a bit on the short side for CERTAIN PELLETS...IF OD is considered BUT very good IF concentrically is concerned. JSB don't engrave but 97 gr RicMor's do slightly. and they both do <3/4" groups.
I finally worked out that my chamber ID is 0.357" +/- a tad...it seats the NSA 105 gr's ≈0.250" INTO the lands...0.358" OD will engrave a nice bore/groove pattern onto those pellets...the 95 gr NSA's will do a bit less and the 75 gr JUST touches the lands.
I'm thinking of installing a LW barrel...if I do then I will throat for whatever pellet I decide to use (and matches the bore/grooves) to just touch then lands but start by throating for a 0.100" engrave and see how it shoots.
Nice presentation...lets hope more shooters start to fiddle with their toys and offer their insights.
Good Shooting
-
Watching this closely. With all of the money that is or can be spent on various mods, casting equipment, high-end scopes, and time spent tinkering this is very intriguing to me. If this relatively simple and economical mod IMPROVES accuracy it would be a huge accomplishment. I like the fact that you used different bullets as examples so as to not limit yourself to a mod that requires a certain size bullet.
I have a .308 Texan and I currently get my best 5 shot groups at about 1.5" at 100 yards and 2.5" at 200 yards. 2" at 100 yards and 3.5" at 200 yards are a reliable consistent average. Please post a follow up when you get some shooting in and have the before and after comparison. Congrats on having the stones to make the modification; it always takes someone to push the envelope to make improvements.
-
The only question I have is what the before and after results are of group sizes and efficiency. This is something I have pondered as well with the .357 slugs sticking out up to 3/8”.
-
I have been thinking about this chambering issue on my .257 Texan and on my Sam Yang 909s. What a pain to get bullets seated the same shot after shot. If a bullet is properly sized for the barrel then you'd think it would slip in to the breach perfectly, and easily every single time. I think that your modification would help with accuracy. If so, sign me up!
-
Great job here. As a recent new Texan owner, I was baffled at first about why slugs did not seat all the way and Airforce made it that way. The leadin definitely should be machined a bit more. I look forward to your results regarding accuracy.
-
And the .457 Pictures
Hard to tell from the barrel pictures as it is foreshortened but that's 0.200 deeper.
The throating reamer is standard SAAMI dimensions, so there is no ridge in the 'After' photo, just shadow on the long taper
The bullets in pictures 5, 7 and 9 are the most accurate
-
Very nice.
-
Let me clarify something:
Do NOT feed the reamer forward with the tailstock unless the barrel/chuck is turning or you will break it.
I just got off the phone with a member who did not grasp that from my written description.
I would have edited the origional post but since there is a lockout time that had expired, I could not do it there and am having to add it down here.
Even gunsmiths using a lathe powered on keep the speed around 100RPM and feed forward slow.
I chose to just hand turn the chuck (gears disengaged) and AS IT IS TURNING simultaneously feed in the reamer mounted in the tailstock. If you can't use both hands to do something, have someone else who can do it for you.