I hope this reaction is known:

`x = 22, y = 22, rule = LifeHistory`

15.D$14.3D$13.2D2BD$13.5B$13.6B$12.8B$13.2B2A4B$14.B2A5B$15.7B$15.6B$

15.2A3B$14.B2A3B$14.6B$14.6B$11.9B$4.2B2.12B$3.C15B$.3BC13B$4B2C12B$

3B2C13B$.2BC13B$2.3B.B!

It seems like it could have some applications.

It can be used to do things like these (obsolete) p539 and p515 guns, where two mutually reset each other in tandem:

`x = 305, y = 127, rule = LifeHistory`

190.2A$190.ABA$190.A3B$191.4B$192.4B$193.4B$194.4B$85.A11.2A96.4B$84.

A.A9.B2AB96.4B$76.A3.2A2.A.A9.3B98.4B$75.A.A2.A2.2A.2A9.B.B98.4B$75.A

.A3.A.A2.B8.5B99.4B$76.A.4A2.AB2A6.6B100.4B$78.A3.A.A.2A4.8B101.4B$

77.A2.BA.A2.13B103.4B$76.A.2BA.A5.13B102.4B$76.2A.2BA5.15B102.4B$51.

4B7.A17.6B.15B103.4B$52.4B6.3A16.4B.17B103.4B$53.4B8.A8.29B104.4B$54.

4B6.2A7.16B2A13B104.4B$55.4B5.5B.B2.16B2A14B104.4B$56.4B6.6B.27B3.B2A

104.4B$57.4B4.34B4.A2.A104.4B$58.25B2.2B2.B3.6B5.2A.A104.4B$59.21B14.

6B7.A3.2A4.B95.4B$59.19B14.9B6.2A2.A.A2.4B94.4B$57.20B15.2A4.4B10.A7B

15.2A78.4B6.3B$56.21B16.A5.4B11.6B8.BA5.A80.4B4.2A3B9.A$56.13B2.8B11.

3A7.4B10.7B3.4BABA.BA.A81.4B3.2A4B6.3A$53.AB.12B5.B3.2A11.A10.4B.B5.

17B2A.B2A83.4B2.7B4.A$52.A.A15B7.A24.31B86.4B2.6B4.2A$53.A2B.B.6B3.2A

8.3A21.31B87.4B.5B2.5B$54.B4.7B2.A11.A21.30B83.2A4.14B$58.7B4.3A30.

28B85.2A5.14B2.B$58.7B6.A31.26B94.14B3A$57.8B36.2AB2.22B87.4A5.17B$

55.2AB.7B34.A.AB3.21B.2B84.A2.A5.18B$54.A.AB.6B35.A5.24B2A80.BA.A2.A.

AB2.14B3A3B$54.A4.6B6.2A26.2A5.24B2A77.B.BA.A4.2AB.14BA2BA4B$53.2A4.

7B6.A32.23B.2B72.A4.2A3BA7.20BA5B$59.8B2.3A33.23B73.3A4.2AB.4B5.15BA

3B2A8B$57.12BA36.20B74.A8.B3.3B5.14B2A3BA10B$57.11B38.16B78.2A10.4B.

19BABA12B$57.5B2A4B38.4B3A8B76.A4B3.30BA8BA5B$57.5B2A3B39.5BA8B75.2BA

2B4.30BA3BA4BA8B$57.10B38.6B3A7B74.2BA4B2.29BA10BA4B.2A$57.9B38.18B

72.9B.29BA4B2A5B5.A$57.9B39.2A16B72.38BA2B2A6B7.3A$58.8B39.2A17B70.

20B2.2B4.11B3A8B9.A$57.8B41.14B.4B68.18B9.3B.21B$58.6B42.11B5.4B66.

17B11.2A3.20B$58.6B43.8B8.4B64.17B13.A3.7B.12B3.A11.2A$58.6B32.2A9.8B

9.4B62.18B10.3A6.4B.13B2.A.A9.B2AB$58.7B16.A15.A9.7B4.2A69.12B.8B8.A

9.2B2.12B3.A.A9.3B$59.6B14.3A15.A.AB5.8B4.A69.13B3.B3.2A17.B2AB.8B6.

2A.2A9.B.B$58.6B14.A19.2AB.8B2A2B.BA.A68.11BA2B7.A19.2A2.8B6.A2.B8.5B

$49.2A8.5B14.2A20.10B2A2B.B2A68.10B2AB2A8.3A21.6B8.AB2A6.6B$49.A9.6B

14.B20.16B69.10BA5B10.A20.7B6.A.A.2A4.8B$46.BA.A9.6B14.3B19.14B69.9B

2A6BAB28.8B6.2A2.13B$46.B2A10.6B13.6B16.16B67.4B.5B2ABA4BA3B19.2A4.8B

13.13B$19.2A23.3B12.7B11.10B11.18B57.B2.21BA2B2A2B2A18.A.A2.9B12.15B$

20.A22.4B12.7B8.B2.11B3.2B2.20B57.20B5A4BA.BA.A19.A2.3B.6B11.15B$20.A

.AB18.4B13.8B6.8B2A31B58.20BABA5BAB4.A19.3A10B8.B.17B$21.2AB.2A14.4B

4.B.7B.22B2A22B.7B57.24BABA3BA2B3.2A16.2A2.BA2.34B$23.2B2AB12.4B.13B.

46B2.6B57.23B2ABABA5B19.A2.4AB.20B2A13B$23.4B6.72B3.6B55.BA14B2.2B2.

2B2AB2A7B19.A.A.3B2.19B2A14B$23.6B3.31B2A2B2A35B6.4B55.ABA10B10.2BA2B

A2B2.4B17.2A.A.2A33B3.B2A$24.6B2.31B2A2B2A9B.13B.4B12.B2A2B55.ABA8B

12.2BA2BAB4.4B16.A2.ABA6B2A25B4.A2.A$24.7B.22B2A22B.7B.B4.4B14.2A.B2A

54.A8B14.2B3A6.4B16.2A4.AB2.B2A9B2.2B2.B3.6B5.2A.A$23.12B3A16B2A8B6.

8B13.4B18.BA.A53.10B14.6B5.4B17.5A3.9B14.6B7.A$22.15BA4B2.2B3.11B2.B

8.7B12.4B22.A54.9B18.2A6.4B10.A5.A7.7B14.9B6.2A25.2B$21.15BA2B11.10B

11.7B12.3B23.2A54.8B18.A8.4B7.3A7.A5.B2A3B15.2A4.4B7.A23.5B$21.16B16.

6B13.6B10.2AB81.7B20.3A6.4B5.A9.2A5.B2A3B5.2A9.A5.4B6.3A20.7B$22.14B

19.3B14.6B9.A.AB82.5B23.A7.4B4.2A15.6B5.A7.3A7.4B8.A18.8B$21.16B20.B

14.6B9.A87.2B25.2A6.9B14.7B7.A5.A10.4B6.2A18.9B$19.2AB.14B20.2A14.5B

8.2A115.A7.6B14.9B3.5A17.4B5.6B14.10B$18.A.AB.12B.B2A19.A14.6B124.A.

2A5.6B3.B2.2B2.12B2.BA4.2A16.4B6.5B14.8BA$18.A4.8B5.BA.A15.3A14.6B

126.A2.A4.33BABA2.A16.4B4.7B12.8BABA$17.2A4.7B9.A15.A16.7B126.2AB3.

33B2A.A.2A17.4B2.8B10.10BABA$9.4B9.8B9.2A32.6B127.14B2A19B2.3B.A.A19.

14B2.2B2.14BAB$10.4B8.8B43.6B128.13B2A20B.B4A2.A19.34B$11.4B5.11B42.

6B129.34B2.AB2.2A16.2A3.33B$12.4B.14B41.8B128.17B.B8.10B3A19.A4.30B$

13.17B2A39.8B130.15B11.6B.3B2.A19.A.AB.30B$14.16B2A39.9B129.15B12.9B

2.A.A18.2A28B2.B$15.18B38.9B130.13B13.8B4.2A19.17B.4B$16.16B38.10B

132.13B2.2A6.8B28.19B$17.14B39.3B2A5B131.8B4.2A.A.A6.7B20.A10.16B$16.

15B38.4B2A5B131.6B6.2ABA8.6B21.3A8.15B$15.16B38.11B131.5B8.B2.A6.2B3A

3B2.2A19.A7.14B$11.20B36.A12B131.B.B9.2A.2A6.4BA3B.B2AB17.2A3.B3.13B$

9.23B33.3A2.8B134.3B9.A.A3.7B3A2B2.2B9.A8.8B.12B$6.2B.23B32.A6.7B4.2A

127.B2AB9.A.A2.13B.4B6.3A10.18B$5.2A24B5.2A26.2A6.6B4.A129.2A11.A3.

12B.7B3.A13.17B$5.2A24B5.A35.6B.BA.A146.20B3.2A11.17B$6.2B.21B3.BA.A

34.7B.B2A147.21B.3B9.18B$9.22B2.B2A36.8B140.A9.22B4.2B2.20B$8.26B31.A

6.7B141.3A7.49B$7.28B30.3A4.7B144.A5.41B.9B$5.30B21.A11.A2.7B4.B139.

2A.45B2.7B$4.31B21.3A8.2A3.6B.B.2BA138.48B4.5B$4.31B24.A7.8B2A5BA.A

139.45B3.5B$2.2AB.2A17B5.B.4B10.A11.2A3.B5.5BA2BA3B.BA139.34B.4B10.2A

$.A.AB.ABA4B3.7B10.4B7.3A11.8B2.9BA3B142.30B5.3B3.B8.A$.A5.AB8.6B11.

4B5.A16.14B3A4B143.29B5.4B.B2A4.3A$2A15.7BA10.4B4.2A15.20B147.26B7.A

3B2A4.A$17.4B2.A.A2.2A6.9B14.19B151.22B.B2A4.A.AB.B$19.B4.2A3.A7.6B

14.21B152.20B2.BA.A2.A.AB$29.A.2A5.6B3.B2.2B2.25B153.18B5.A2.A$30.A2.

A4.34B4.BABA153.17B5.4A$31.2AB3.27B.6B6.B2AB153.17B$32.14B2A16B2.B.5B

5.A3B153.B2.14B5.2A$33.13B2A16B7.2A6.4B156.14B4.2A$34.29B8.A8.4B153.

5B2.5B.4B$34.17B.4B16.3A6.4B152.2A4.6B2.4B$35.15B.6B17.A7.4B152.A4.7B

2.4B$35.15B5.A2B.2A174.3A6.4B2A3.4B$36.13B5.A.A2B.A174.A9.3B2A4.4B$

38.13B2.A.AB2.A186.3B6.4B$37.8B4.2A.A.A3.A$37.6B6.2ABA2.4A.A$37.5B8.B

2.A.A3.A.A$37.B.B9.2A.2A2.A2.A.A$38.3B9.A.A2.2A3.A$37.B2AB9.A.A$38.2A

11.A!

It can also put a B in very tight spaces, due to its extremely thin reaction envelope (5 cells at least, 8 or 9 cells at most) such as Scorbie's thumb B-to-H:

`x = 22, y = 31, rule = LifeHistory`

15.B$14.3D$14.BDB$13.2B3D$13.6B$12.7B$11.8B$4.2A4.8B$4.A.A2.9B$6.A2.

3B.2BD3B$6.3A5B3D2B$4.2A2.BA2.B2D2BD$3.A2.4AB.6B$4.A.A.3B2.6B$3.2A.A.

2A10B$3.A2.ABA6B2A4B$4.2A4.AB2.B2A5B$6.5A4.7B$6.A8.6B$8.A6.2A3B$7.2A

5.B2A3B$14.6B$14.6B$11.9B$4.2B2.12B$3.C15B$.3BC13B$4B2C12B$3B2C13B$.

2BC13B$2.3B.B!

EDIT: Something Similar:

`x = 43, y = 31, rule = LifeHistory`

23.3B13.4B$22.5B11.4B$22.5B10.4B$9.B12.5B9.4B$8.3B10.7B7.4B$4.7B7.2B.

8B5.4B$3.10B3.12B5.4B$4.9B2.13B4.4B$4.24B3.4B$3.24B3.4B$3.30B$2.30B$

2.29B$3.28B$3.7B2A3B2A14B$2.8B2A3B2A14B$3.9B2.17B$3.7B4.20B$.9B3.22B$

.2BC6B3.22B$3BCBC4B3.22B$.2B3C4B3.18B2D2B$.4BC4B5.6B.10B2DB$2.7B8.2B

5.8BD3B$4.B19.12B$25.10B$26.8B$26.7B$26.5B$27.3B$28.B!

Unfortunately, an extra block and blinker are produced. There might be a way to clean it up with the r's exhaust, though...