If it's harmonic, you can experiment with retuning it to shoot the required load.
Wind several turns of copper electrical wire (jnsulated/uninsulated, it doesn't matter; it's the weight we're concerned with here) around the muzzle end. Begin trimming the wire and shooting, while keeping track of dispersion.
When you get to where it begins to open up again; stop, take the last piece you had cut off and halve it. Then mark the front and rear lines where the copper wire rides with masking tape, and weigh one of the halved pieces along with the remaining coils. A reloading scale would be good for this.
Go to the auto parts store and buy some stick-on wheel weights. Trim one down to where its weight is identical to the wire weight. Stick it to the bottom of the barrel, centered between the two masking tape lines. If it still shoots true, remove it and epoxy it in place.
This should return your barrel to tune.
Me, I'd wait until I had permanently mounted the brake first.
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Once, I built a wooden balance beam scale for measuring rubber powered indoor free flight model aircraft and parts. Many of these entire planes weighed as little as 2 grams. The beam was made of balsa and was two feet long, The balance weights were made by balancing 1 cc of water against a length of fine copper wire, deriving a 1 gram weight. Cut maybe ten more, same length. Take a couple and cut the wire in half, 1/2 gram, in quarters, .25gm, and so on down to about .0125 gm.
These planes were flown for duration indoors, some managing over 30 minutes. My daughter set a National Youth Penny Plane (must weigh more than one penny) duration record at age 17. Not easy, but the real competition was among the bearded old guys who had been doing this same thing for decades. We flew in The Columbia University Lowe Memorial Rotunda on Morningside Heights, in Manhattan, for one Sunday a month. The Rotunda was 80 ft across, hexagonal, and 105 ft tall. I sponsored the Bill Tyler Memorial Trophy for
Manhattan Cabin designs (basically an ROG - 'Rise Off Ground' Penny Plane with a Cabin Fuselage).
The strategy was to build successive versions of the same plane design, while incrementally building each major component a bit lighter in weight each time. Eventually parts failed. You'd then build the next one using A) the previous part that didn't fail, and B) the other parts built a bit lighter. Eventually you had your competition plane, because anything lighter would fall apart. They flew at a slow walking speed.
You can't stand up and watch the longer lights, your neck cramps up on you. We'd all bring a pillow and lie back flat on the floor to watch. The sport has been going on for a large several score years, and is still going strong; although venues are disappearing. We lost Columbia in the '80's or early '90's due to Nanny liability caution.
Something for the real addicts...; they are just prototype testing, and are only winding the rubber part way That fuselage motor stick is a tube rolled up around a mandrel from balsa wood sanded down to about 1/64", or less. If you can't read newsprint through it, the balsa sheet is too heavy.
There's a bit more to my world than just the guns...
Greg
