UPDATE: Corsair+ 7mm (part 2)

To say this Air gun is coming along nicely is an understatement.  Its been a lot of trial and error with this gun and I believe that is what is to be expected when you transform any weapons platform from one state to another.  But the gun is now coming to its final stage of modifying it and I'm getting the desired results I was originally hoping for.  Ultimate long range accuracy for target shooting and hunting.  That IS the purpose of this gun!

HISTORY (of the gun):  The gun started life as a standard 250 FPE Corsair .308 which is wholly based off the Chinese QB78 platform.    The brainchild of the gun was Randy Mitchell and his concept was to build a relatively cheap mid bore PCP rifle that would be readily available and everyone could afford ($600). He then sourced out a builder for the project.  That person is Dan McVey over at XP airguns.  The main parts of the gun that were to be altered from OEM QB was the barrel and the airtube.  The stock airtube was ditched and the replacement was to be made out of 4130 Chrome Moly.  Even the guns stock was to remain OEM.  Randy and Dan succeeded in making a no frills and inexpensive mid bore airgun.  I then ditched the OEM stock and added a Richards Microfit Stock in thumb-hole configuration to the gun to spice it up a tad!

CURRENT (the gun):  Today, the Corsair has gone through some obvious changes.  Let's go into detail about them.  Lets start with the receiver.  The gun now wears a higher tensile strength aluminum breech (7075-T6) than the OEM version.  Hence the name Corsair+.  This is needed for the repeated high fill pressures which requires the much need strength.  The OEM weaver rails were scrapped due to a better source of material and better fitment to the receiver by my machinist Doug Noble.  The OEM flow through bolt probe was replaced by Dan McVey due to excessive were from the air blast.  The one on the gun now is a standard probe also made out of a better grade of steel than the OEM version.  The valve and valve body have gone unchanged from the OEM 308 version but the valve stem was sanded down and reprofiled.  The barrel has gone through a few phases.  1st, in OEM form, it was a 28" .308 barrel.  Then I switched that to a 7mm at 1-14" twist then I went to a 7mm 1-10" twist thinking that this would be best for the long for caliber bullets.  This thought was also due to the LEE RN bullets not being very accurate past 150 yards.  I was wrong on both counts.  So now, today, the Corsair+ 7mm wears the 1-14" twist that I originally had on the gun.  I find that with this platform of only 250 FPE, the 10" twist barrel really dropped the FPS down by a great margin.  A loss of 60 fps with the same bullets I originally shot in the 14" twist was the norm.  This was unacceptable.  And the accuracy was not good either.  So I switched back to the 14" twist that is 30.25" long and has of O.D. of .562".  It has 6 lands and grooves and slugs at .2852".  It is also threaded at the end with 1/2 x 20 TPI and wears a muzzle break that I once had installed on my Remington SP 308.  The barrel is also now Carbon Fiber sleeved for extra rigidity and aided accuracy.  And boy has it helped!  The CF sleeve was bonded to the barrel via the same Acraglas epoxy product that is used to glass bed powder burner rifles.  The trigger is of standard QB configuration and is set to about 1.5 lbs+-.  The stock is from Michael Chavka who no longer makes gun stocks unfortunately.

TETHERED vs UNTETHERED:  Today, I shoot the gun tethered and unless i'm hunting with it, it stays tethered.  Mid/Big Bore air guns are air hogs because they make so much more power than a small bore air gun (.177-.25 cal).  And many small bore air guns have regulators which makes the gun much more efficient on air as well as more accurate.  It makes the gun super consistent.  Well, its the same principle except we are using the size of our refill SCBA tanks as our air reservoir.  With out this type setup, many big bores will only get 2-3 shot MAX from a normal fill.  The gun is tethered and or topped off at 3600 PSI.

BULLET MOLD of CHOICE:  I have shot 2 LEE bullets in this gun.  One is the "SOUPCAN" 130 gr. FP bullet and the other is a LEE 130 gr RN bullet.  The LEE RN shot decently but I noticed at ranges further than 150 yards, the bullet was not a great performer.  But that makes sense, its a round nose. The "SOUPCAN" bullet was simply not a performer at all in this gun.  The other MAJOR thing with these 2 bullet molds was the fact that they were horribly made by LEE.  Well, at least fitment of the 2 mold halves were sloppy and the 2 halves were sloppy in fitting the mold handles.  So in comes the LBT mold.  Specifically, it is the LBT 287-130 mold.  Which means, the bullets drop out of the mold at .287" and weigh 130 grs.  Veral Smith owns and operates LBT Bullet Molds and make some of the nicest and most accurate molds I laid eye upon.  My LBT 7mm mold is of the 2 cavity format and one side of the mold is hollow pointed by Eric at Hollowpointbulletmoldservices.com.  This has dropped the weight from FP form by only about 4 grs.  The FP version weighs roughly 133 grs and the HP version goes for 129 gr.  The HP are the ones that posted the shown group.  The HP pin does not go down deep into the bullets body (midway down ogive) but it goes down and is wide enough for ultra reliable expansion and will allow the bullet to drive deep into game.  There is also a new 7mm custom designed bullet created by Bob Sterne over on the Gateway To Airguns forum that will be in my possession soon.  The mold will be made by LBT also and will be roughly 95grs+- AND Boat Tailed and was specifically designed for the 1-14" twist.  That should offer more speed and a better BC due to the lighter weight and Boat Tail.  So stay tuned on its performance in this gun.  I'm expecting great things from that bullet.

 

GROUPS:  While i've shot various groups with this gun, the accuracy that I was originally expecting is starting to really rear its beautiful head.  My original want was for at least MOA at every range that I shot the gun.  So, .5" at 50 yards, 1" at 100 yards, 2" at 200 yards and so forth.  So far, the gun is staying BELOW MOA at these ranges which is much better than what I expected.  Let's take a closer look at these groups:

This group was shot this past weekend.  These were with bullets that were weight sorted to within .5 a grain and sized the night before I shot this group.  This process of bullet sorting and weighing will be the new process from here on out.  Without putting real pressure on myself, this is the new standard that I will expect from my gun OR BETTER!

This group on the steel rabbit was shot at 431 yards and the group measures a measly 3.420" CtC.  There is a video of this quick shoot on my Youtube channel.  The group was also shot using UNSORTED and UNWEIGHTED bullets.  So as you can see, this gun has some serious potential once bullets are sorted and weighed for consistency!

 This groups was shot late last year with the new mold and unweighted and sorted bullets.  Still under MOA and the sizing process that I now used was NOT used during the group! Nor did the gun wear the Carbon Fiber sleeve that it does today.

This groups was shot the same day as the group above except it was done at 55 yards and I tried different fill pressures along the way.  The bullets were also unweighted and sorted.  The .176" group was more than likely due to me grabbing 3 bullets that weighed exactly the same or very close to it.  These bullets like the group just above were sized about a MONTH or more before this shoot.  This is NOT the process I will be performing as stated above.  The reason being is that i have learned that certain tin to lead mixtures will make or let the bullet grow the longer it sits on the shelf.  Some of the bullets from the day would not fit inside the breech and could not be loaded even though they were sized a couple of months ago.

In closing, the Corsair+ 7mm with all its mods and attention to detail are starting to really pay off and show the extreme potential of what a modern PCP airgun can do.  This gun in its current configuration cost no more than a high end powder burner and is showing long range accuracy on par with many of them.  Long range shooting is challenging and when you throw an airgun in the mix, things get even more interesting really quick.  Its not easy but it sure is fun and it provides as level of satisfaction that you will have to achieve yourself to fully comprehend.

Stay tuned!

Tofazfou